Silicon on insulator and thin film transistor bandgap engineered split gate memory

ABSTRACT

Memory cells comprising thin film transistor, stacked arrays, employing bandgap engineered tunneling layers in a junction free, NAND configuration. The cells comprise a channel region in a semiconductor strip formed on an insulating layer; a tunnel dielectric structure disposed above the channel region, the tunnel dielectric structure comprising a multilayer structure including at least one layer having a hole-tunneling barrier height lower than that at the interface with the channel region; a charge storage layer disposed above the tunnel dielectric structure; an insulating layer disposed above the charge storage layer; and a gate electrode disposed above the insulating layer Arrays and methods of operation are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/056,489, filed Mar. 27, 2008. The U.S. application Ser. No.12/056,489 is a continuation-in-part of U.S. patent application Ser. No.11/831,594, filed Jul. 31, 2007; which is a continuation of U.S. patentapplication Ser. No. 11/324,581, filed Jan. 3, 2006, which is basedupon, and claims priority under 35 U.S.C. §119(e) of provisional U.S.Patent Application No. 60/640,229, filed on Jan. 3, 2005; provisionalU.S. Patent Application No. 60/647,012, filed on Jan. 27, 2005;provisional U.S. Patent Application No. 60/689,231, filed on Jun. 10,2005; and provisional U.S. patent application No. 60/689,314, filed onJun. 10, 2005; the entire contents of each of which are incorporatedherein by reference. The U.S. application Ser. No. 12/056,489 is acontinuation-in-part of U.S. patent application Ser. No. 11/425,959,filed on Jun. 22, 2006, which claims priority to provisional U.S. PatentApplication No. 60/748,807, filed on Dec. 9, 2005, the entire contentsof each of which are incorporated herein by reference. The U.S.application Ser. No. 12/056,489 is a continuation-in-part of U.S. patentapplication Ser. No. 11/549,520, filed on Oct. 13, 2006, which claimspriority to provisional U.S. Patent Application No. 60/748,911, filed onDec. 9, 2005, the entire contents of each of which are incorporatedherein by reference. The U.S. application Ser. No. 12/056,489 claims thebenefit of provisional U.S. Patent Application No. 60/980,788, filed onOct. 18, 2007 and provisional U.S. Patent Application No. 61/018,589,filed on Jan. 2, 2008, the entire contents of each of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

Non-volatile memory (“NVM”) refers to semiconductor memory which is ableto continually store information even when the supply of electricity isremoved from the device containing the NVM cell. NVM includes MaskRead-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM),Erasable Programmable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), and Flash Memory. Non-volatilememory is extensively used in the semiconductor industry and is a classof memory developed to prevent loss of programmed data. Typically,non-volatile memory can be programmed, read and/or erased based on thedevice's end-use requirements, and the programmed data can be stored fora long period of time.

Generally, non-volatile memory devices may have various designs. Oneexample of an NVM cell design is the so-called SONOS(silicon-oxide-nitride-oxide-silicon) device, which may use a thintunnel oxide layer, to allow hole direct tunneling erase operations.Although such designs may have good erase speed, the data retention isusually poor, in part because direct tunneling may occur even at a lowelectrical field strengths that may exist during a retention state of amemory device.

Another NVM design is NROM (nitrided read-only memory), which uses athicker tunnel oxide layer to prevent charge loss during retentionstates. However, a thick tunnel oxide layer may impact channel erasespeed. As a result, band-to-band tunneling hot-hole (BTBTHH) erasemethods can be used to inject hole traps to compensate the electrons.However, the BTBTHH erase methods may cause some reliability issues. Forexample, the characteristics of NROM devices employing BTBTHH erasemethods may degrade after numerous P/E (program/erase) cycles.

In addition, techniques have been explored to stack layers of memoryarrays on a single integrated circuit in order to address the need forhigh-density non-volatile memory.

Thus, a need in the art exists for non-volatile memory cell designs andarrays which can be operated (programmed/erased/read) numerous timeswith improved data retention performance and increased operation speeds,and in addition are suitable for implementation in thin film structuresand in stacked arrays.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to junction-free, thin-film memory cellsformed on silicon on insulator substrates and similar insulatingstructures, and to stacked junction free memory cells. An integratedcircuit memory device is described comprising a semiconductor bodyformed on an insulating layer, such as on a silicon on insulatorsubstrate; a plurality of gates arranged in series on the semiconductorbody, the plurality of gates including a first gate in the series and alast gate in the series, with insulating members isolating gates in theseries from adjacent gates in the series; and a charge storage structureon the semiconductor body. The charge storage structure includesdielectric charge trapping locations beneath more than one of theplurality of gates in the series, the charge storage structure includinga tunnel dielectric structure disposed above the semiconductor body, acharge storage layer disposed above the tunnel dielectric structure, andan insulating layer disposed above the charge storage layer. Thesemiconductor body includes a continuous, junction-free, multiple-gatechannel region beneath the plurality of gates in the series. Themultiple-gate channel region may have one of n-type and p-typeconductivity.

One embodiment of the present invention includes memory cellscomprising: a semiconductor substrate having a source region and a drainregion disposed below a surface of the substrate and separated by achannel region; a tunnel dielectric structure disposed above the channelregion, the tunnel dielectric structure having a hole tunneling barrierheight at an interface with the semiconductor body, and a hole tunnelingbarrier height spaced away from the interface that is less than the holetunneling barrier height at an interface. A tunnel dielectric layerhaving this characteristic comprises a multi-layer structure including alayer in contact with the semiconductor body and at least one layerhaving a hole-tunneling-barrier height less than that of the layer incontact with the semiconductor body. A charge storage layer disposedabove the tunnel dielectric structure; an insulating layer disposedabove the charge storage layer; and a gate electrode disposed above theinsulating layer.

Another embodiment of the present invention includes memory cells incontrast to the junction-free embodiments, comprising a semiconductorsubstrate having a source region and a drain region disposed below asurface of the substrate and separated by a channel region; amulti-layer tunnel dielectric structure disposed above the channelregion, the multi-layer tunnel dielectric structure comprising at leastone layer having a hole-tunneling-barrier height less than that of thelayer in contact with the semiconductor body; a charge storage layerdisposed above the multi-layer tunnel dielectric structure; aninsulating layer disposed above the charge storage layer; and a gateelectrode disposed above the insulating layer.

In certain preferred embodiments, the layer providing a smallerhole-tunneling-barrier height may contain materials such as siliconnitride (Si₃N₄) or hafnium oxide (HfO₂). In certain preferredembodiments of the present invention memory cells include a tunneldielectric structure having multiple layers, such as a stackeddielectric tri-layer structure of silicon oxide, silicon nitride, andsilicon oxide (ONO). Such tunnel dielectric structures provide a SONONOS(silicon-oxide-nitride-oxide-nitride-oxide-silicon) or a super-latticeSONONOS design.

In certain preferred embodiments of the present invention the tunneldielectric structure can comprise at least two dielectric layers eachhaving a thickness of up to about 4 nm. Additionally, in certainpreferred embodiments of the present invention, the gate electrodecomprises a material having a work function value greater than that ofN⁺ polysilicon.

In certain preferred embodiments, the tunnel dielectric structure caninclude a layer comprising a material having a small hole tunnelingbarrier height, wherein the material is present in the layer at aconcentration gradient such that the concentration of the material is ata maximum at a depth point within the layer.

The present invention also includes non-volatile memory devices whichcomprise a plurality of memory cells (i.e., an array) in accordance withone or more of the embodiments described herein. As used herein, a“plurality” refers to two or more. Memory devices in accordance with thepresent invention exhibit significantly improved operational propertiesincluding increased erase speeds, improved charge retention and largerwindows of operation.

The present invention also includes methods of operating non-volatilememory cells and arrays. Methods of operation in accordance with thepresent invention include resetting the memory devices by applying aself-converging method to tighten Vt distribution of the memory devices;programming at least one of the memory devices by channel +FN injection;and reading at least one of the memory devices by applying a voltagebetween an erased state level and a programmed state level of at leastone of the memory devices. As used herein, the term “tighten” refers tothe narrowing of the threshold voltage distribution among the manymemory cells of an array. In general, threshold voltage distribution is“tightened” where the threshold voltages of several cells are within anarrow range of one another such that operation of the array is improvedover conventional designs. For example, in some preferred embodiments,such as in a NAND array comprising memory cells in accordance with oneor more embodiments of the present invention, a “tightened” thresholdvoltage distribution indicates that the threshold voltages of thevarious memory cells are within a 0.5V range of one another. In otherarray architectures employing memory cells in accordance with thepresent invention, the “tightened” threshold voltage distribution mayhave a range of about 1.0V from the upper limit to the lower limit.

One embodiment of a method of operation in accordance with the presentinvention includes operating an array in accordance with the presentinvention by applying self-converging reset/erase voltages to thesubstrate and the gate electrode in each memory cell to be reset/erased;programming at least one of the plurality of memory cells; and readingat least one of the plurality of memory cells by applying a voltagebetween an erased state level and a programmed state level of at leastone of the memory devices.

The present invention also includes methods of forming a memory cell,comprising: providing a semiconductor substrate having a source regionand a drain region formed therein below a surface of the substrate andseparated by a channel region; forming a tunnel dielectric structureabove the channel region, wherein forming the tunnel dielectricstructure comprises forming at least two dielectric layers, wherein oneof the at least two dielectric layers has a smaller hole tunnelingbarrier height than the other of the at least two dielectric layers;forming a charge storage layer above the tunnel dielectric structure;forming an insulating layer above the charge storage layer; and forminga gate electrode above the insulating layer.

According to an exemplary embodiment of junction-free technology, asemiconductor structure includes a plurality of first parallelsemiconductor semiconductor body regions over a silicon-on-insulatorsubstrate, the plurality of first semiconductor body regions beingcharacterized by a first concentration of a first dopant type. A firstselect line and a second select line overlie and are substantiallyperpendicular to the first semiconductor body regions. A plurality offirst parallel word lines are between the first select line and thesecond select line, each of the plurality of first word lines overlyinga channel region in each of the first semiconductor body regions andbeing substantially perpendicular to the first semiconductor bodyregions. A first tunneling barrier, a first charge storage layer, and afirst dielectric layer are between each of the first word lines and acorresponding channel region in each of the first semiconductor bodyregions. At least one first region is in each of the first semiconductorbody regions. The at least one first region is adjacent to the firstselect line or the second select line. The at least one first region ischaracterized by a second dopant type. One or more second regions are ineach of the first semiconductor body regions, each of the one or moresecond regions being between two neighboring channel regions, the one ormore second regions being characterized by a second concentration of thefirst dopant type, wherein the one or more second regions arejunction-free.

According to an exemplary embodiment of this SOI technology, thesemiconductor structure further comprises a plurality of trenchstructures adjacent to and in parallel with the first semiconductor bodyregions, each of the trench structures separating two adjacent firstsemiconductor body regions.

According to an exemplary embodiment of this SOI technology, the firsttunneling barrier includes a first oxide layer, a nitride layer and asecond oxide layer.

According to an exemplary embodiment of this SOI technology, the firsttunneling barrier, the first charge storage layer, and the firstdielectric layer is an ONONO structure.

According to an exemplary embodiment of this SOI technology, the SOIsubstrate comprises an oxide layer over the substrate and under thefirst semiconductor body regions.

According to an exemplary embodiment of this SOI technology, the firstregion extends under at least one of the first select line and thesecond select line.

According to an exemplary embodiment of this SOI technology, thesemiconductor structure is stacked providing multiple layers ofjunction-free, memory cells, such that it further comprises: a seconddielectric layer over the first word lines. A plurality of secondparallel semiconductor body regions with a third concentration of thefirst dopant type overlie the second dielectric layer. A plurality ofsecond parallel word lines are between a third select line and a fourthselect line, the second word lines, the third select line and the fourthselect line being over and substantially perpendicular to the secondsemiconductor body regions. A second tunneling barrier, a second chargestorage layer and a second dielectric layer are between the second wordlines and the second semiconductor body regions. The secondsemiconductor body regions include at least one third region adjacent tothe third select line and the fourth select line and fourth regionsbetween two neighboring second word lines. The fourth regions arecharacterized with a fourth concentration of the first dopant type. Adimension of the first region is larger than that of the third region.

It is to be noted that in stacked, junction-free embodiments, the bottomlayer can be implemented on a SOI substrate, or directly on asemiconductor bulk region, without an overlying layer of insulation.

According to another exemplary embodiment of this technology disclosedherein, a method for forming a semiconductor structure comprises forminga plurality of first parallel semiconductor body regions having a firstconductivity type over a substrate. A first select line, a second selectline and a plurality of first parallel word lines are formed over andsubstantially perpendicular to the first semiconductor body regions, theword lines configured between the first select line and the secondselect line. A first tunneling barrier, a first charge storage layer anda first dielectric layer are formed between the first semiconductor bodyregions and the word lines. First dielectric spacers are formed on asidewall of the first select line and a sidewall of the second selectline, while forming first dielectric materials between two neighboringword lines. First source/drain (S/D) regions having a secondconductivity type are formed adjacent to the first select line and thesecond select line by using the first dielectric spacers as animplantation mask. A region is formed between two neighboring wordlines. The region between neighboring word lines has a secondconcentration of the first type, wherein the region between twoneighboring word lines is substantially junction-free.

According to an exemplary embodiment of this application, a method foroperating a semiconductor structure is provided. The semiconductorstructure comprises: a plurality of parallel semiconductor body regionsover a substrate; a plurality of parallel word lines between a firstselect line and a second select line, the word lines including aselected word line and a plurality of unselected word lines, the wordlines, the first select line and the second select line being over andsubstantially perpendicular to the semiconductor body regions; and atunneling barrier, a charge storage layer and a dielectric layer betweenthe word lines and the semiconductor body regions, wherein thesemiconductor body regions include at least one first region adjacent tothe first select line and the second select line and second regionsbetween two neighboring word lines, wherein the first region has adopant concentration higher than that of the second regions and whereinat least one of the second regions is junction-free. The methodcomprises applying a first voltage to the first select line and thesecond select line; applying a second voltage to the word lines, thefirst voltage being higher than the second voltage; and applying a thirdvoltage to the semiconductor body regions to reset the semiconductorstructure, the third voltage being higher than the second voltage.

As used herein, the phrase “small hole tunneling barrier height” refersgenerally to values which are less than the approximate hole tunnelingbarrier height at a silicon dioxide/silicon interface. In particular, asmall hole tunneling barrier height is preferably less than about 4.5eV. More preferably, a small hole tunneling barrier height is less thanor equal to about 1.9 eV.

A junction-free TFT NAND device for multiple stackable 3D Flash memoryis proposed. The TFT NAND has no diffusion junction (such as N+-dopedjunction) in the memory array. Diffusion junctions are only fabricatedoutside the array select transistors BLT and SLT.

An inversion layer will be induced by the wordline fringing field whenthe space between each wordline is small (for example, a 75 nm node).The junction-free TFT NAND structure avoids the junction punch throughafter repeating thermal budget. Short-channel effect can be suppressed.Thus this technique enables multiple short stacks of TFT NAND structure,achieving very high density.

3D Flash memory has attracted a lot of attention recently. 3D multiplestacks of memory enables much higher density than the conventionalsingle-layer memory devices.

Traditional doped junction (such as n+ doped junction) has a largelateral diffusion after thermal process. The lateral diffusion isserious for very short channel device. The short-channel effect becomesmore serious for a 3D Flash with multiple stacks of TFT NAND devices.The bottom layers have much larger thermal budgets so that lateraldiffusion of the junction causes a severe punch through, which seriouslydegrades the short channel effect performance.

The junction-free NAND described herein enables multiple stacks andjunction only diffuses at the array boundary, which offers a largeprocess window to avoid punch-through.

Unlike conventional devices where the junction is formed before thespacers, a method for manufacturing the junction-free TFT NAND includesforming the junction after the spacers between the wordlines are formed.The spacer between each wordline is completely filled without a gap dueto the small pitch of the TFT NAND array. Therefore, the junction IMP isblocked by the spacers inside the memory array, and junctions areinstead formed outside the array.

In an alternative method one additional mask is introduced whichoverlays the wordlines and the BLT and SLT, and the junction IMP iscarried out.

Simulation results show that an inversion layer can be inducedunderlying the spacers due to the fringing field from the high electricfield on the wordlines, such that there is no need to fabricate n+-dopedregions.

The devices described herein also include p-channel TFT NAND, wheren-well and P+ junction are used.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIGS. 1 a and 1 b are cross-sectional schematic representations of anN-channel memory cell in accordance with one embodiment of the presentinvention and a P-channel memory cell in accordance with one embodimentof the present invention, respectively;

FIG. 2 is a graphical representation of the threshold voltage (chargetrapping capacity) of a tunnel dielectric structure in accordance withone embodiment of the present invention under various programmingmethods;

FIG. 3 is a graphical representation of the threshold voltage of aSONONOS memory cell in accordance with one embodiment of the presentinvention over time during erase;

FIG. 4 is a graphical representation of the threshold voltage of aSONONOS memory cell in accordance with one embodiment of the presentinvention over time during retention;

FIGS. 5 a-5 e are band energy diagrams of ONO tunnel dielectricstructures in accordance with various embodiments of the presentinvention;

FIG. 6 is a graphical representation of hole-tunneling current versuselectrical field strength for three different tunnel dielectricstructures;

FIG. 7 a is a graphical representation of the threshold voltage overtime of a memory cell in accordance with one embodiment of the presentinvention during erase after various types of programming;

FIG. 7 b is a graphical representation of the threshold voltage overtime of a memory cell having a platinum gate in accordance with oneembodiment of the present invention during erase;

FIGS. 7 c and 7 d are graphical representations of capacitance versusvoltage for the memory cell referred to in FIG. 7 b;

FIG. 8 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under variousoperating conditions;

FIG. 9 is a graphical representation of the current-voltage (IV)relationship for a memory cell in accordance with one embodiment of theinvention after one cycle and 10³ cycles;

FIG. 10 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under one set ofprogramming and erasing conditions;

FIG. 11 is a graphical representation of the change in threshold voltageover time in a memory cell according to one embodiment of the presentinvention under VG-accelerated retention testing;

FIGS. 12 a and 12 b are an equivalent circuit diagram and layout view,respectively, of a virtual ground array of memory cells in accordancewith one embodiment of the present invention;

FIG. 13 is a cross-section schematic representation of a virtual groundarray of memory cells in accordance with one embodiment of the presentinvention taken along line 12B-12B as shown in FIG. 12 b;

FIGS. 14 a and 14 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention and depicting suitable reset/erase voltages in accordance withtwo embodiments of operation in accordance with the present invention;

FIGS. 15 a and 15 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention depicting one method of programming in accordance with thepresent invention;

FIGS. 16 a and 16 b are equivalent circuit diagrams of memory arrayscomprising memory cells in accordance with one embodiment of the presentinvention depicting one method of reading a bit in accordance with thepresent invention;

FIG. 17 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover time under various erasing conditions;

FIG. 18 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles;

FIGS. 19 a and 19 b are graphical representations of the current at thedrain of a memory cell in accordance with one embodiment under variousgate voltages depicted in a logarithmic scale and a linear scale,respectively;

FIG. 20 is an equivalent circuit diagram of an array including memorycells in accordance with one embodiment of the present inventiondepicting one method of programming a bit in accordance with the presentinvention;

FIGS. 21 a and 21 b are a layout view and equivalent circuit diagram ofa virtual ground array in accordance with one embodiment of the presentinvention;

FIGS. 22 a and 22 b are an equivalent circuit diagram and layout view,respectively, of a NAND array of memory cells in accordance with oneembodiment of the present invention;

FIGS. 23 a and 23 b are cross-sectional schematic representations of aNAND array of memory cells in accordance with one embodiment of thepresent invention taken along lines 22A-22A and 22B-22B, respectively,as shown in FIG. 22 b;

FIG. 24 a is an equivalent circuit diagram of a NAND array in accordancewith one embodiment of the present invention depicting one method ofoperation in accordance with the present invention;

FIG. 24 b is a graphical representation of threshold voltages over timeduring a reset operation in accordance with one embodiment of thepresent invention for two memory cells having different initialthreshold voltages;

FIG. 25 is an equivalent circuit diagram depicting a method of operationin accordance with one embodiment of the present invention;

FIG. 26 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover time under various erasing conditions;

FIG. 27 is an equivalent circuit diagram depicting a method of operationin accordance with one embodiment of the present invention;

FIG. 28 is a graphical representation of the threshold voltage of amemory cell in accordance with one embodiment of the present inventionover the course of numerous programs/erase cycles under one set ofprogramming and erasing conditions

FIGS. 29 a and 29 b are graphical representations of the current at thedrain of a memory cell in accordance with one embodiment under variousgate voltages at three different cycle numbers depicted in a logarithmicscale and a linear scale, respectively

FIG. 30 is a graphical representation of the threshold voltage of memorycells in accordance with one embodiment of the present invention overtime during retention at three different temperature and cycleconditions;

FIG. 31 is a cross-sectional schematic representation of a NAND arraywordline in accordance with one embodiment of the present invention; and

FIG. 32 is a cross-sectional schematic representation of a NAND arraywordline formation technique in accordance with one embodiment of thepresent invention.

FIG. 33 is a graph of the change in threshold voltage versus the numberof programming pulses for an nMOSFET having an ONO tunneling dielectricfor a number of programming bias arrangements.

FIG. 34 a graph of the change in voltage versus time under negativecurrent stress for a capacitor having an ONO tunneling dielectricinsulator.

FIG. 35 is a graph of the self-convergent threshold voltage versus oferase gate voltage.

FIG. 36 illustrates endurance of a memory cell as described herein, withhigh-temperature baking of a device in accordance with an embodiment.

FIG. 37 illustrates change in flat band voltage versus erase time for−FN programming bias levels in a device in accordance with anembodiment.

FIG. 38 illustrates change in flat band voltage versus program time for+FN programming bias levels in a device in accordance with anembodiment.

FIG. 39 illustrates the P/E cycle endurance of a device in accordancewith an embodiment.

FIG. 40 illustrates an accelerated retention test of a device inaccordance with an embodiment.

FIG. 41 illustrates the charge retention in the charge trapping nitrideN2 at room temperature and high temperature of a device in accordancewith an embodiment.

FIG. 42 illustrates the erase characteristics of devices of varyingdimensions in accordance with embodiments.

FIG. 43 illustrates the erase characteristics of devices of various gatematerial in accordance with embodiments.

FIG. 44 is a schematic top view showing a portion of an exemplary memoryarray for a thin-film transistor, charge trapping memory array.

FIG. 45 is a schematic cross-sectional view of a portion of an exemplaryarray taken along the section line 2-2 of FIG. 44 for a thin-filmtransistor, charge trapping memory.

FIGS. 46A and 46B are schematic cross-sectional view showing anexemplary semiconductor structure taken along section line 3-3 of FIG.44 for a thin-film transistor, charge trapping memory.

FIG. 46C is a schematic cross-sectional view showing an exemplaryprocess for implanting dopants within semiconductor body regions for athin-film transistor, charge trapping memory.

FIG. 47 is a schematic cross-sectional views showing a portion of anexemplary stacked structure for a thin-film transistor, charge trappingmemory.

FIG. 48 is a schematic cross-sectional view showing an exemplary processfor generating an inversion layer in a semiconductor body region for athin-film transistor, charge trapping memory.

FIGS. 49A-49B are drawings showing simulations of electron density ofexemplary junction-free BE-SONOS NAND implemented with a thin-filmtransistor, charge trapping memory.

FIG. 50 is a figure showing the measured initial IV curve of exemplaryn-channel devices for a thin-film transistor, charge trapping memory.

FIG. 51 shows that a heavier well dopant concentration can increase theVt of the junction-free device.

FIGS. 52A-52B are drawings showing +FN ISPP programming and −FN erasing,respectively, for a thin-film transistor, charge trapping memory.

FIG. 53 is a drawing showing electrical characteristics of an exemplaryP-channel BE-SONOS NAND having a stack structure similar to theN-channel BE-SONOS NAND described above in conjunction with FIG. 50.

FIG. 54A is a graph of threshold voltage versus program voltage for a−FN ISPP programming.

FIG. 54B is a graph showing erase time versus threshold voltage for a+FN erase.

FIG. 55 is a drawing showing endurance of exemplary n-channel devicesfor a thin-film transistor, charge trapping memory.

FIG. 56 is a drawing showing IV curve of exemplary TFT BE-SONOS devicesfor a thin-film transistor, charge trapping memory.

FIG. 57 is a drawing showing simulations of exemplary junction-freedevices having various technology nodes (F=half pitch of poly), andhaving same spaces (S=20 nm) for a thin-film transistor, charge trappingmemory.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the invention and the presentlypreferred embodiments thereof, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same or similar referencenumbers are used in the drawings and the description to refer to thesame or like parts. It should be noted that the non-graph drawings arein greatly simplified form and are not to precise scale. In reference tothe disclosure herein, for purposes of convenience and clarity only,directional terms, such as top, bottom, left, right, up, down, above,below, beneath, rear, and front, are used with respect to theaccompanying drawings. Such directional terms used in conjunction withthe following description of the drawings should not be construed tolimit the scope of the invention in any manner not explicitly set forthin the appended claims. Although the disclosure herein refers to certainillustrated embodiments, it is to be understood that these embodimentsare presented by way of example and not by way of limitation. It is tobe understood and appreciated that the process steps and structuresdescribed herein do not cover a complete process flow for themanufacture of entire integrated circuits. The present invention may bepracticed in conjunction with various integrated circuit fabricationtechniques that are known in the art or to be developed.

Memory cells in accordance with the present invention can overcome someof the reliability issues in SONOS and NROM devices. For example, memorycell structures in accordance with the present invention may allow fastFN channel erase methods, while at the same time, maintaining goodcharge retention characteristics. Various embodiments of the memorycells according to the present invention can also alleviate reliance onthe BTBTHH erase method, thereby avoiding device degradation afternumerous P/E cycles.

One example may employ an ultra-thin tunnel dielectric or ultra-thinoxide layer in conjunction with the small hole tunneling barrier heightlayer in embodiments where the tunnel dielectric structure is amultilayer structure. This may provide better stress immunity.Non-volatile memory cells according to the present invention also showlittle degradation after numerous P/E cycles.

Memory cells according to the present invention may employ either ann-channel or a p-channel design, such as shown in FIGS. 1 a and 1 b.FIG. 1 a depicts a cross-sectional view of an n-channel memory cell 100in accordance with one embodiment of the present invention. The memorycell includes a p-type substrate 101 containing at least two n-dopedregions 102 & 104, wherein each of the doped regions 102 & 104 mayfunction as either a source or drain depending upon voltages applied. Asshown in FIG. 1 a, for reference purposes, doped region 102 can serve asthe source and doped region 104 can serve as the drain. The substrate101 further includes a channel region 106 between the two n-dopedregions. Above the channel region 106, on the surface of the substrate101, is a tunnel dielectric structure 120. In certain preferredembodiments, the tunnel dielectric structure 120 can comprise atri-layer thin ONO structure wherein a small hole-tunneling-barrierheight nitride layer 124 is sandwiched between a thin lower oxide layer122 and an upper thin oxide layer 126. The memory cell 100 furtherincludes a charge-trapping (or charge storage) layer 130, preferably anitride, above the tunnel dielectric structure 120, and an insulatinglayer 140, preferably comprising a blocking oxide, disposed above thecharge-trapping layer 130. A gate 150 is disposed on the insulatinglayer 140.

FIG. 1 b, depicts a cross-sectional view of an p-channel memory cell 200in accordance with one embodiment of the present invention. The memorycell includes an n-type substrate 201 containing at least two p-dopedregions 202 & 204, wherein each of the doped regions 202 & 204 mayfunction as either a source or drain. The substrate 201 further includesa channel region 206 between the two p-doped regions. The p-channelmemory cell 200 similarly includes a tunnel dielectric structure 220,comprising a tri-layer thin ONO structure wherein a smallhole-tunneling-barrier height nitride layer 224 is sandwiched between athin lower oxide layer 222 and an upper thin oxide layer 226, acharge-trapping (or charge storage) layer 230, an insulating layer 240,and a gate 250.

Thus, for example, as depicted in FIGS. 1 a and 1 b, memory cells inaccordance with the present invention may include: a multi-layer thinfilm tunnel dielectric structure, including a first silicon oxide layerO1, a first silicon nitride layer N1, and a second silicon oxide layerO2; a charge-storage layer, such as a second silicon nitride layer N2;and an insulating layer such as a third silicon oxide layer O3, on orover (“above”) a substrate, such as a semiconductor substrate (e.g., asilicon substrate). The tunneling dielectric structure allows holetunneling from the substrate to the charge-storage layer during anerase/reset operation of the memory device. Preferably, the tunneldielectric structure in a non-volatile memory cell of the presentinvention has a negligible charge-trapping efficiency, and morepreferably, does not trap charge at all during memory operations.

Charge storage materials such as a silicon nitride layer, HfO₂, andAl₂O₃ may be used as the small hole tunneling barrier height layer in atunnel dielectric structure. In certain preferred embodiments of thepresent invention, an efficient charge storage material, such as asilicon nitride can be used as a charge storage layer in the memorydevice. A blocking oxide that prevents charge loss may serve as aninsulating layer, such as a third silicon oxide layer O3. The memorycells according to the present invention also include a gate or gateelectrode, such as a polysilicon gate, above the insulating layer. Thetunnel dielectric structure, charge storage layer, insulating layer andgate can be formed above the substrate above at least a portion of achannel region, which is defined by and is disposed between a sourceregion and a drain region.

Memory cells according to various embodiments of the present inventioncomprise a tunnel dielectric structure which can provide fast FN erasespeeds of around 10 msec under a negative gate voltage (Vg), such as aVg of about −10 to about −20 V. On the other hand, the charge retentioncan still be maintained, and, in some examples, may be better than manyconventional SONOS devices. Memory cells according to the presentinvention can also avoid the use of band-to-band hot hole eraseoperations, which, are commonly used in NROM devices. Avoidance of suchband-to-band hot hole erase operations may greatly eliminate hot-holeintroduced damages and such avoidance is therefore desirable.

Referring to FIG. 2, experimental measurements of threshold voltage fora tunnel dielectric structure in accordance with one embodiment of thepresent invention shows that an ultra-thin O1/N1/O2 structure can have anegligible trapping efficiency, as evidenced by the unchanged thresholdvoltage level under successive programming pulses. In the example testedfor FIG. 2, the O1/N1/O2 layers had thicknesses of 30, 30 and 35angstroms (Å), respectively. As shown in FIG. 2, the threshold voltageVt remains steady at approximately 1.9 volts over the course of severalprogram shots using various methods of programming, namely −FNprogramming, +FN programming and CHE (channel hot electron) programming.Thus, such an ultra thin O1/N1/O2 film may serve as a modulated tunneldielectric structure because charge trapping appears to be negligible inthe structure having nitride layer of 30 Angstroms or less. The resultsunder various charge injection methods including CHE, +FN and −FN allsuggest negligible charge trapping. Manufacturing processes or devicestructures may be designed to minimize interfacial traps, so thatneither O1/N1 nor N1/O2 interface is active.

FIG. 3 illustrates the erase characteristics of a memory cell having aSONONOS design in accordance with one embodiment of the presentinvention. The memory cell in the embodiment described in FIG. 3comprises an n-MOSFET design with an ONO tunnel dielectric structurehaving thicknesses of 15 Å, 20 Å and 18 Å, respectively. The memory cellof this embodiment comprises a silicon nitride charge storage layerhaving a thickness of about 70 Å, an insulating silicon oxide layer witha thickness of about 90 Å, and a gate comprising any suitable conductivematerial, for example, n-doped polycrystalline silicon. Referring toFIG. 3, fast FN erase may be achieved, such as within 10 msec, and anexcellent self-convergent erase properties may also be obtained.

FIG. 4 illustrates the charge retention characteristics of a SONONOSdevice in accordance with an embodiment of a memory cell according tothe present invention as described with reference to FIG. 3. As shown,the retention characteristics can be better than those of conventionalSONOS devices, and in terms of magnitude, may be many orders better.

FIGS. 5 a and 5 b are band diagrams which illustrate possible effects ofusing a tunnel dielectric structure containing at least one layer havinga small hole-tunneling-barrier height. The band diagram of the tunneldielectric structure, an O1/N1/O2 trilayer in this example, under a lowelectrical field, which may exist during memory data retention, is shownin FIG. 5 a. Direct tunneling as represented by the dotted arrow may beeliminated under low electrical fields, thereby providing good chargeretention during retention states. On the other hand, band diagramoffset under a high electrical field, as shown in FIG. 5 b, can reducethe barrier effect of N1 and O2 such that the direct tunneling throughO1 may occur. A tunnel dielectric structure having at least one smallhole-tunneling-barrier height layer may allow efficient FN eraseoperation.

FIGS. 5 c and 5 d illustrate another set of band diagrams in oneexample. For a better band offset condition in one example, thethickness of N1 may be larger than that of O1. The band diagram ofvalence band is plotted at the same electrical field E01=14 MV/cm. Thetunneling probability according to WKB approximation is correlated tothe shadow area. In this example, for N1=O1 in thickness, the bandoffset does not completely screen out the barrier of O2. On the otherhand, for N1>O1, the band offset can more easily screen out O1.Therefore, for N1>O1 in thickness, the hole tunneling current may belarger under the same electrical field in O1.

An experiment with measured and simulated hole tunneling currents, asshown in FIG. 6, further describes hole tunneling through a tunneldielectric structure according to certain embodiments of the presentinvention. For example, hole tunneling current through the O1/N1/O2dielectric may fall between that of an ultra-thin oxide and a thickoxide. In one example, under a high electrical field, the hole tunnelingcurrent may approximate that of an ultra-thin oxide. However, under alow electrical field, the direct tunneling can be suppressed. As shownin FIG. 6, hole tunneling current is detected through a thin oxide layereven at low electrical field strengths of only 1 MV/cm. Hole tunnelingcurrent is negligible through a thick oxide even at relatively highfield strengths such as, for example, 11-13 mV/cm. However, holetunneling current through an ONO tunnel dielectric structure approachesthat of a thin oxide layer when high electric field strengths arepresent. In FIG. 6, the large current leakage due to hole tunnelingthrough an ultra-thin oxide at low electrical fields can be seen at areaA of the graph. In FIG. 6, hole tunneling current through an O1/N1/O2tunnel dielectric structure at high electric field strengths can be seenat area B of the graph. In FIG. 6, the virtually non-existent tunnelingcurrent through an O1/N1/O2 tunnel dielectric structure and a thickoxide at low electrical fields can be seen at area C of the graph.

Memory cell designs in accordance with the present invention may beapplied to various memory types, including but not limited to, NORand/or NAND-type flash memories.

As noted above, a tunnel dielectric layer may include two or morelayers, including one layer that may provide a smallhole-tunneling-barrier height. In one example, the layer providing asmall hole-tunneling-barrier height may contain silicon nitride. Thelayer may be sandwiched between two silicon oxide layers, therebyforming an O/N/O tunnel dielectric if silicon nitride is used as theintermediate layer. In some preferred embodiments, the bottom layer canhave a thickness from about 2 nm or less. The middle and top layers inthe tunnel dielectric structure can have a thickness of about 1 nm to 3nm. In one exemplary device, a tri-layer structure may have a bottomlayer, such as a silicon oxide layer, of about 10 Å to 20 Å, anintermediate layer, such as a silicon nitride layer, of about 10 Å to 30Å, and a top layer, such as another silicon oxide layer, of about 10 Åto 30 Å. In one particular example, an O/N/O tri-layer structure havinga 15 Å bottom silicon oxide layer, a 20 Å intermediate silicon nitridelayer, and an 18 Å top silicon oxide layer may be used. In oneparticular example, an O/N/O tri-layer structure having a 13 Å bottomsilicon oxide layer, a 25 Å intermediate silicon nitride layer, and an25 Å top silicon oxide layer may be used.

In one example, a thin O/N/O tri-layer structure shows negligible chargetrapping. Theoretical band diagram and tunneling current analysis, suchas described with reference to FIGS. 5 a, 5 b and 6, may suggest that atunnel dielectric structure, such as an O1/N1/O2 structure havingthicknesses of 3 nm or less for each of the layers, can suppress thehole direct-tunneling at low electric field during retention. At thesame time, it still may allow efficient hole tunneling at high electricfield. This may be because the band offset can effectively screen outthe tunneling barrier of N1 and O2. Therefore, this proposed device mayoffer fast hole tunneling erase, while it is immune from the retentionproblem of the conventional SONOS devices. Experimental analysis showsexcellent endurance and retention properties of memory cells inaccordance with various embodiments of the present invention.

In certain preferred embodiments, the tunnel dielectric structureincludes at least a middle layer and two adjacent layers on opposingsides of the middle layer, wherein each of the middle layer and twoadjacent layers comprises a first material and a second material,wherein the second material has a valence band energy level greater thanthe valence band energy level of the first material and the secondmaterial has a conduction band energy level less than the conductionband energy level of the first material; and wherein the concentrationof the second material is higher in the middle layer than in the twoadjacent layers and the concentration of the first material is higher inthe two adjacent layers than in the middle layer. Preferably, in atunnel dielectric structure in accordance with this embodiment of thepresent invention, the first material comprises oxygen and/or anoxygen-containing compound and the second material comprises nitrogenand/or a nitrogen-containing compound. For example, the first materialcan comprise an oxide, such as silicon oxide, and the second materialcan comprise a nitride, such as Si₃N₄ or Si_(x)O_(y)N_(z).

Tunnel dielectrics in accordance with this aspect of the invention maybe comprised of three or more layers, all of which can contain similarelements (such as Si, N and O), so long as the concentration of thematerial having the smallest hole tunneling barrier height is higherwithin the middle layer than in the two adjacent layers.

In certain tunnel dielectric structures according to the precedingembodiment of the present invention, the second material can be presentin the middle layer in a gradient concentration such that theconcentration of the second material in the middle layer increases fromone adjacent layer/middle layer interface to a maximum concentration ata depth point within the middle layer, and decreases from the maximumconcentration depth point to a lower concentration at the other adjacentlayer/middle layer interface. The increase and decrease in concentrationis preferably gradual.

In still other embodiments of the present invention, the tunneldielectric structure includes at least a middle layer and two adjacentlayers on opposing sides of the middle layer, wherein the two adjacentlayers comprise a first material and the middle layer comprises a secondmaterial, wherein the second material has a valence band energy levelgreater than the valence band energy level of the first material and thesecond material has a conduction band energy level less than theconduction band energy level of the first material; and wherein thesecond material is present in the middle layer in a gradientconcentration such that the concentration of the second material in themiddle layer increases from one adjacent layer/middle layer interface toa maximum concentration at a depth point within the middle layer, anddecreases from the maximum concentration depth point to a lowerconcentration at the other adjacent layer/middle layer interface. Theincrease and decrease in concentration is preferably gradual.Preferably, in a tunnel dielectric structure in accordance with thisembodiment of the present invention, the first material comprises oxygenand/or an oxygen-containing compound and the second material comprisesnitrogen and/or a nitrogen-containing compound. For example, the firstmaterial can comprise an oxide, such as silicon oxide, and the secondmaterial can comprise a nitride, such as Si₃N₄ or Si_(x)O_(y)N_(z).

For example, in embodiments of the present invention where the tunneldielectric layer comprises a tri-layer ONO structure, the bottom oxideand top oxide layers can comprise silicon dioxide and the middle nitridelayer can be comprised of, for example, silicon oxynitride and siliconnitride wherein the concentration of silicon nitride (i.e., the materialhaving the smaller hole tunneling barrier height of the two) is notconstant within the layer, but rather reaches a maximum at some depthpoint within the layer between the two interfaces with the sandwichingoxide layers.

The precise point within the middle layer where the material with thesmallest hole tunneling barrier height reaches its maximum concentrationis not critical, so long as it is present in a gradient and reaches itsmaximum concentration in the tunnel dielectric layer at some pointwithin the middle layer.

The gradient concentration of the material having the smallest holetunneling barrier height can be advantageous in improving variousproperties of non-volatile memory devices, particularly those having aSONONOS, or SONONOS-like structure. For example, retention state chargeloss can be diminished, hole tunneling under high electric fields can beimproved and, to the extent it may occur, charge-trapping in the tunneldielectric can be avoided.

The band diagram of a tunnel dielectric layer can be advantageouslymodified in accordance with this aspect of the present invention suchthat the valence band energy level and the conduction band energy levelof the middle layer do not have a constant value, but rather vary acrossthe thickness of the layer with the concentration of the material havingthe smallest hole tunneling barrier height. Referring to FIG. 5 e,modification of an ONO tri-layer tunnel dielectric in accordance withthis aspect of the invention is shown via a band diagram. The middlelayer (Layer-2) is comprised of silicon nitride. The outer layers(Layer-1 and Layer-3) are comprised of silicon dioxide. Theconcentration of silicon nitride in Layer-2 is varied such that thevalence band energy level and the conduction band energy level reach amaximum and minimum value, respectively, at the depth in Layer-2 wherethe concentration of silicon nitride is highest. Three possible siliconnitride concentration gradients are shown in FIG. 5 e, depicted bydashed lines representing the variable valence band energy conductionband energy levels that result from the concentration gradients. Asshown in FIG. 5 e, by the circles on the dashed lines representing threealternative silicon nitride concentration maximums within Layer-2, thelowest valence band energy level and the highest conduction band energylevel coincide with the silicon nitride concentration maximum.

Multi-layer tunnel dielectric structures in accordance with suchembodiments of the present invention, can be prepared in a variety ofways. For example, a first silicon dioxide or silicon oxynitride layercan be formed using any number of conventional oxidation approachesincluding, but not limited to thermal oxidation, radical (ISSG)oxidation, and plasma oxidation/nitridation, as well as chemical vapordeposition processes. A middle layer with a gradient concentration ofSiN can then be formed, for example, via chemical vapor depositionprocesses, or alternatively, by plasma nitridation of excess oxide oroxynitride formed on top of the first layer. A third layer, the upperoxide layer, can then be formed, for example, by oxidation or chemicalvapor deposition.

A charge storage layer can then be formed over the tunnel dielectricstructure. In one example, a charge storage layer of about 5 nm to 10 nmmay be formed over the tunnel dielectric structure. In one particularexample, a silicon nitride layer of about 7 nm or thicker may be used.The insulating layer above the charge storage layer may be about 5 nm to12 nm. For example, a silicon oxide layer of about 9 nm or thicker maybe used. And the silicon oxide layer may be formed by a thermal processconverting at least a portion of a nitride layer to form the siliconoxide layer. Any method, known or to be developed, for forming layers ofsuitable materials described herein can be used to deposit or formtunnel dielectric layers, charge-storage layers and/or insulatinglayers. Suitable methods include, for example, thermal growth methodsand chemical vapor deposition methods.

In one example, a thermal conversion process may provide a high densityor concentration of interfacial traps that can enhance the trappingefficiency of a memory device. For example, thermal conversion ofnitride can be carried out at 1000, while the gate flow ratio isH2:O2=1000:4000 sccm.

In addition, because silicon nitride generally has very low (about 1.9eV) hole barrier, it may become transparent to hole tunneling under highfield. Meanwhile, the total thickness of a tunnel dielectric, such as anONO structure, may prevent direct tunneling of electrons under a lowelectric field. In one example, this asymmetrical behavior may provide amemory device offering not only fast hole-tunneling erase, but alsoreduction or elimination of charge leakage during retention.

An exemplary device may be fabricated by 0.12 μm NROM/NBit technologies.Table 1 shows the device structure and parameters in one example. Theproposed tunnel dielectric with an ultra-thin O/N/O may alter the holetunneling current. A thicker (7 nm) N2 layer may serve as acharge-trapping layer and an O3 (9 nm) layer may serve as the blockinglayer in one example. Both N2 and O3 may be fabricated using NROM/NBittechnologies.

TABLE 1 Layer Approximate Thickness (Angstroms) Bottom Oxide (O1) 15Inter Nitride (N1) 20 Inter Oxide (O2) 18 Trapping Nitride (N2) 70Blocking Oxide (O3) 90 Gate: N+-polysilicon Channel length: 0.22 μmChannel width: 0.16 μm

In certain embodiments of the present invention, a gate can comprise amaterial having a work function greater than that of N⁺ polysilicon. Incertain preferred embodiments of the present invention, such a high workfunction gate material can comprise a metal such as, for example,platinum, iridium, tungsten, and other noble metals. Preferably, thegate material in such embodiments has a work function greater than orequal to about 4.5 eV. In particularly preferred embodiments, the gatematerial comprises a high work function metal such as, for example,platinum or iridium. Additionally, preferred high work functionmaterials include, but are not limited to P⁺ polysilicon, and metalnitrides such as, for example, titanium nitride and tantalum nitride. Inparticularly preferred embodiments of the present invention, the gatematerial comprises platinum.

An exemplary device in accordance with an embodiment of the presentinvention having a high work function gate material may also befabricated by 0.12 μm NROM/NBit technologies. Table 2 shows the devicestructure and parameters in one example. The proposed tunnel dielectricwith an ultra-thin O/N/O may alter the hole tunneling current. A thicker(7 nm) N2 layer may serve as a charge-trapping layer and an O3 (9 nm)layer may serve as the blocking layer in one example. Both N2 and O3 maybe fabricated using NROM/NBit technologies.

TABLE 2 Layer Approximate Thickness (Angstroms) Bottom Oxide 15 InterNitride 20 Inter Oxide 18 Trapping Nitride (N2) 70 Blocking Oxide 90Gate: Platinum Channel length: 0.22 μm Channel width: 0.16 μm

Memory cells in accordance with high work function gate materialembodiments of the present invention exhibit erase properties which areeven more improved over other embodiments. High work function gatematerials suppress gate electron injection into the trapping layer. Incertain embodiments of the present invention wherein the memory cellscomprise an N⁺ polysilicon gate, hole tunneling into the charge-trappinglayer during erase occurs simultaneously with gate electron injection.This self-converging erase effect results in higher threshold voltagelevels in the erased state, which can be undesirable in NANDapplications. Memory cells in accordance with high work function gatematerial embodiments of the present invention can be used in varioustype of memory applications including, for example, NOR- and NAND-typememories. However, the memory cells according to high work function gatematerial embodiments of the present invention are particularly suitablefor use in NAND applications where elevated threshold voltages in theerased/reset state can be undesirable. Memory cells in accordance withhigh work function gate material embodiments of the present inventioncan be erased via hole tunneling methods and preferably via −FN erasingoperations.

An exemplary device having an ONO tunneling dielectric and an N+polysilicon gate may be programmed by conventional SONOS or NROM methodand erased by channel FN hole tunneling. FIG. 7 a shows the erasecharacteristics of an exemplary SONONOS device having an ONO tunnelingdielectric in one example. Referring to FIG. 7 a, a higher gate voltageresults in a faster erase speed. It also has a higher saturation Vt,because gate injection is also stronger and the resulting dynamicbalance point (which determines the Vt) is higher. This is shown on theright-hand side of the graph as the threshold voltage reaches a minimumat values of from about 3 to about 5 volts depending upon the erase gatevoltage. The hole tunneling current can be extracted by a transientanalysis method by differentiating the curves in FIG. 7 a. The extractedhole current from the measurement in FIG. 7 a is illustrated in FIG. 6as discussed above. For comparison, there is also plotted simulated holetunneling current using WKB approximation. The experimental results arein reasonable agreement with our prediction. The tunneling currentthrough the O1/N1/O2 stack approaches that of the ultra-thin O1 under ahigh electric field, while it is turned-off under a low electric field.

In accordance with certain embodiments of memory cells of the presentinvention having high work function gate materials, wherein the highwork function gate suppresses gate electron injection, the thresholdvoltage of the device in an erased or reset state can be much lower, andeven negative, depending upon erase time. The threshold voltage valuesof a memory device in accordance with one embodiment of the presentinvention wherein the gate is comprised of platinum and the tunneldielectric layer comprises a 15/20/18 angstrom ONO structure are shownin FIG. 7 b. As shown in FIG. 7 b, at a similar gate voltage (−18 V)during a-FN erase operation, the flat band voltage (which correlateswith threshold voltage) of the device can be set below −3V. Thecorresponding capacitance versus gate voltage values for the device areshown in FIG. 7 c.

Moreover, retention properties of memory devices in accordance with highwork function gate material embodiments the present invention areimproved. The retention properties of a memory device having a platinumgate are shown in FIG. 7 d wherein the capacitance is graphed as afunction of gate voltage following erase and program, and then 30minutes after each operation and two hours after each operation. Minimaldeviation is observed.

Memory cells in accordance with various embodiments of the presentinvention may be operated with at least two separate schemes. Forexample, CHE programming with reverse read (mode 1) may be used toperform a 2-bits/cell operation. Additionally, low-power +FN programming(mode 2) may also be used for a 2-bits/cell operation. Both modes canuse the same hole tunneling erase method. Mode 1 may preferably be usedfor a virtual ground array architecture for NOR-type flash memories.Mode 2 may preferably be used for NAND-type flash memories.

As an example, FIG. 8 shows the excellent endurance properties of avirtual ground array architecture NOR-type flash memory in accordancewith one embodiment of the present invention under mode 1 operation.Erase degradation of such memory devices having a tunnel dielectricstructure does not occur, because hole tunneling erase (Vg=−15 V) is auniform channel erase method. The corresponding IV curves are also shownin FIG. 9, which suggest little degradation of the device after numerousP/E cycles. In one example, this may be because ultra-thin oxide/nitridelayers possess good stress immunity properties. Additionally, the memorydevice is free of hot-hole introduced damages. The endurance propertiesof a NAND-type flash memory in accordance with one embodiment of thepresent invention under Mode 2 operation are shown in FIG. 10. For afaster convergent erasing time, one may use a larger bias (Vg=−16 V).Excellent endurance may also be obtained in this example.

The charge retention of an exemplary SONONOS device in accordance withone embodiment of the present invention is shown in FIG. 4, where only a60 mV charge loss is observed after 100 hours. The improvement ofretention is many orders of magnitude better than conventional SONOSdevices. VG-accelerated retention test also shows that direct tunnelingcan be suppressed at the low electrical field. FIG. 11 illustrates anexample of a VG-accelerated retention test for a 10K P/E cycled device.The charge loss is small at −VG stress after a 1000 sec stress,indicating that the hole direct tunneling at small electrical field canbe suppressed.

Accordingly, the SONONOS design identified in the above examples mayprovide a fast hole tunneling erase with excellent endurance properties.As noted above, the design may be implemented in both NOR and NAND-typenitride-storage flash memories. Additionally, a memory array inaccordance with the present invention may include multiple memorydevices with similar or different configurations.

In various embodiments of arrays according to the present invention,memory cells according to the present invention may be used in place ofconventional NROM or SONOS devices in a virtual ground arrayarchitecture. The reliability problems and erase degradations may besolved or mitigated by using FN hole tunneling instead of hot-holeinjection. Without limiting the scope of the invention to the specificstructures described below, various operation methods in accordance withmemory arrays of the present invention are described below for exemplaryNOR virtual ground array architectures.

CHE or CHISEL (channel initiated secondary electron) programming andreverse read may be used for 2-bit/cell memory array. And the erasemethod may be a uniform channel FN hole tunneling erase. In one example,the array architecture may be a virtual ground array or a JTOX array.With reference to FIGS. 12 a-20, an O1/N1/O2 tri-layer structure may beused as the tunnel dielectric, the O1 layer having thickness less than 2nm and the N1 and O2 layers having about 3 nm or less in thickness toprovide hole direct tunneling. With reference to FIGS. 12 a-20, N2 maybe thicker than 5 nm to provide a high trapping efficiency. Aninsulating layer, O3, may be a silicon oxide layer formed by wetoxidation, such as a wet converted top oxide (silicon oxide), to providea large density of traps at the interface between O3 and N2. O3 may beabout 6 nm or thicker to prevent charge loss from this silicon oxidelayer.

FIGS. 12 a and 12 b illustrate an example of a virtual ground arrayarchitecture incorporating the memory cells discussed above, such asmemory cells having a tri-layer ONO tunnel dielectric. In particular,FIG. 12 a illustrates an equivalent circuit of a portion of a memoryarray, and FIG. 12 b illustrates an exemplary layout of a portion of thememory array.

In addition, FIG. 13 illustrates a schematic diagram of thecross-sectional view of several memory cells incorporated in the array.In one example, the buried diffusion (BD) areas may be N+-dopedjunctions for the source or drain regions of the memory cells. Thesubstrate may be a p-type substrate. In order to avoid possiblebreakdown of the BDOX areas (oxide above BD) during −FN erase, a thickBDOX (>50 nm) may be used in one example.

FIGS. 14 a and 14 b illustrate possible electrical RESET schemes for anexemplary virtual ground array incorporating 2 bits/cell memory cellshaving a tunnel dielectric design discussed above. Before performingfurther P/E cycles, all the devices may first undergo an electrical“RESET”. A RESET process may ensure the Vt uniformity of memory cells inthe same array and raise the device Vt to the convergent erased state.For example, applying Vg=−15 V for 1 sec, as shown in FIG. 14 a, mayhave the effect of injecting some charge into a charge trapping layer ofsilicon nitride to reach a dynamic balancing condition. With the RESET,even memory cells that are non-uniformly charged due, for example, tothe plasma charging effect during their fabrication processes may havetheir Vt converged. An alternative way for creating a self-convergingbias condition is to provide bias for both gate and substrate voltages.For example, referring to FIG. 14 b, Vg=−8 V and P-well=+7 V may beapplied.

FIGS. 15 a and 15 b illustrate programming schemes for an exemplaryvirtual ground array incorporating 2 bits/cell memory cells having atunnel dielectric design discussed above. Channel hot-electron (CHE)programming may be used to program the device. For Bit-1 programmingillustrated in FIG. 15 a, the electrons are locally injected into thejunction edge above BLN (bit line N). For Bit-2 programming shown inFIG. 15 b, the electrons are stored above BLN-1. Typical programmingvoltage for WL (word line) is around 6 V to 12 V. Typical programmingvoltage for BL (bit line) is about 3 to 7 V, and the p-well may be keptgrounded.

FIGS. 16 a and 16 b illustrate reading schemes for an exemplary virtualground array incorporating 2 bits/cell memory cells having a tunneldielectric design discussed above. In one example, reverse read is usedto read the device to perform a 2 bits/cell operation. Referring to FIG.16 a, for reading Bit-1, BLN-1 is applied with a suitable read voltage,such as 1.6 V. Referring to FIG. 16 b, for reading bit-2, BLN is appliedwith a suitable read voltage, such as 1.6V. In one example, the readingvoltage may be in the range of about 1 to 2 V. The word lines and theP-well may be kept grounded. However, other modified read schemes, suchas a raised-Vs reverse read method can be performed. For example, araised-Vs reverse read method may use Vd/Vs=1.8/0.2 V for reading Bit-2,and Vd/Vs=0.2/1.8 for reading Bit-1.

FIGS. 14 a and 14 b also illustrate sector erase schemes for anexemplary virtual ground array incorporating 2 bits/cell memory cellshaving a tunnel dielectric design discussed above. In one example,sector erase with channel hole tunneling erase may applied to erase thememory cells simultaneously. An ONO tunnel dielectric in a memory cellhaving the SONONOS structure may offer a fast erase, which may occur inabout 10 to 50 msec and a self-convergent channel erase speed. In oneexample, a sector erase operation condition may be similar to a RESETprocess. For example, referring to FIG. 14 a, applying VG=about −15 V atthe WL's simultaneously and leaving all the BL's floating may achieve asector erase. And the p-well may be kept grounded.

Alternatively, referring to FIG. 14 b, applying about −8 V to the WL'sand about +7 V to the p-well may also achieve a sector erase. In someexamples, a complete sector erase operation may be carried out within100 msec or less without having any over-erase or hard-to-erase cells.The device design discussed above may facilitate a channel eraseproviding excellent self-converging properties.

FIG. 17 illustrates the erase characteristics in one example of using anSONONOS device. An example of an SONONOS device may have the thicknessof O1/N1/O2/N2/O3 respectively as about 15/20/18/70/90 Angstroms, withan N+-polysilicon gate and thermally converted top oxide as O3. Theerase speeds for various gate voltages are shown. The erase operation onthe cell having the O1/N1/O2 tunnel dielectric with layers havingthicknesses respectively as about 15/20/18 Angstroms, results in areduction of the threshold voltage of about 2 volts in less than 50msec, for example about 10 msec, under the conditions shown for −FNerase voltages between −15 and −17 volts. A higher gate voltage resultsin a faster erase speed.

However, the convergent Vt is also higher. This is because gateinjection is more active under higher gate voltages. To reduce gateinjection, P+-polysilicon gate or other metal gate with a high workfunction may be used alternatively as the gate material to reduce thegate-injected electrons during the erase.

FIG. 18 illustrates the endurance properties of using SONONOS devices ina virtual ground array architecture. The endurance properties of in someexamples are excellent. The programming condition is Vg/Vd=8.5/4.4 V,0.1 μsec for Bit-1 and Vg/Vs=8.5/4.6 V, 0.1 μsec for Bit-2. The FN erasemay use Vg=−15 V for about 50 msec to erase the two bits simultaneously.Because the FN erase is self-convergent uniform channel erase,hard-to-erase or over-erase cells usually do not present. In someexamples, the devices proposed above show excellent endurance propertieseven without using a Program/Erase verifying or stepping algorithm.

FIGS. 19 a and 19 b illustrate I-V characteristics during P/E cycles inone example. The corresponding I-V curves in both log scale (FIG. 19 a)and linear scale (FIG. 19 b) are shown. In one example, an SONONOSdevice possesses little degradations after numerous P/E cycles, suchthat both the sub-threshold swing (S.S.) and trans-conductance (gm) arealmost the same after numerous cycles. This SONONOS device possessessuperior endurance properties than NROM device. One reason may be thathot-hole injection is not used. Additionally, an ultra-thin oxide asnoted above may possess better stress immunity properties than a thicktunnel oxide.

FIG. 20 illustrates a CHISEL programming scheme in one example. Analternative way to program the device is to use CHISEL programmingscheme, which uses negative substrate bias enhanced impact ionization toincrease the hot carrier efficiency. The programming current can be alsoreduced due to the body effect. Typical condition is illustrated in thisfigure, where substrate is applied with a negative voltage (−2 V), andthe junction voltage is reduced to about 3.5 V. For conventional NROMdevices and technologies, CHISEL programming is not applicable becauseit may inject more electrons near the channel center region. Andhot-hole erase is inefficient to remove the electrons near the channelcenter region in the conventional NROM devices.

FIGS. 21 a and 21 b illustrate the design of a JTOX virtual ground arrayin one example. The JTOX virtual ground array provides an alternativeimplementation of using SONONOS memory cells in a memory array. In oneexample, one difference between the JTOX structure and a virtual groundarray is that the devices in the JTOX structure that are isolated by STIprocesses. A typical layout example is illustrated in FIG. 21 a. FIG. 21b illustrates a corresponding equivalent circuit, which is the same asthat of a virtual ground array.

As noted above, memory cell structures in accordance with the presentinvention are suitable for both NOR- and NAND-type flash memories. Thefollowing will describe additional examples of memory array designs andtheir operation methods. Without limiting the scope of the invention tothe specific structures described below, various operation methods inaccordance with memory arrays of the present invention are describedbelow for exemplary NAND architectures.

As noted above, n-channel SONONOS memory devices having an ONO tunnelingdielectric may be used in a memory device. FIGS. 22 a and 22 billustrate an example of a NAND array architecture. FIGS. 23 a and 23 billustrate the cross-sectional views of an exemplary memory array designfrom two different directions. In some examples, the operation methodsof a memory array may include +FN programming, −FN erase, and readingmethods. Additionally, circuit operation methods may be included toavoid program disturb in some examples.

In addition to the single-block gate structure design, a split-gatearray, such as a NAND array using SONONOS devices positioned between twotransistor gates which are located next to the source/drain regions, mayalso be used. In some examples, a split-gate design may scale downdevice dimension to F=30 nm or below. Furthermore, the devices may bedesigned to obtain good reliability, to reduce or eliminate theinter-floating-gate coupling effect, or to achieve both. As discussedabove, an SONONOS memory device may provide excellent self-converging,or high speed erase, which may help sector-erase operations and Vtdistribution control. Furthermore, a tightened erased state distributionmay facilitate multi-level applications (MLC).

By using certain designs for a memory array structure, the effectivechannel length (Leff) may be enlarged to reduce or eliminateshort-channel effects. Some examples may be designed to use no diffusionjunctions, thereby avoiding the challenges in providing shallowjunctions or using pocket implantations during the manufacturingprocesses of memory devices.

FIG. 1 illustrates an example of a memory device having an SONONOSdesign. In addition, Table 1 noted above illustrates an example ofmaterials used for different layers and their thicknesses. In someexamples, P+-polysilicon gate may be used to provide a lower saturatedReset/Erase Vt, which may be achieved by reducing gate injection.

FIGS. 22 a and 22 b illustrate an example of a memory array, such as anSONONOS-NAND array having memory cells in accordance with embodimentdescribed in Table 1, with diffusion junctions. In one example, separatedevices may be isolated from each other by various isolation techniques,such as by using shallow-trench isolation (STI) or the isolationtechnique of silicon-on-insulator (SOI). Referring to FIG. 22 a, amemory array may include multiple bit lines, such as BL1 and BL2, andmultiple word lines, such as WL1, WLN-1, and WLN. Additionally, thearray may include source line transistor(s) (or source-line-selectingtransistor(s) or SLTs) and bit line transistor(s) (or bit-line-selectingtransistor(s) or BLTs). As illustrated, the memory cells in the arraymay use an SONONOS design, and the SLT and BLT may include n-typemetal-oxide-semiconductor field-effect transistors (NMOSFETs).

FIG. 22 b illustrates an exemplary layout of a memory array, such as aNAND array. Referring to FIG. 22 b, Lg is the channel length of memorycells, and Ls is the space between each separate lines of memorydevices. Additionally, W is the channel width of memory cells, and Ws isthe width of isolation areas between separate bit lines or source/drainareas, which may be the STI width in one example.

Referring again to FIGS. 22 a and 22 b, the memory devices may beconnected in series and form a NAND array. For example, a string ofmemory devices may include 16 or 32 memory devices, providing a stringnumber of 16 or 32. The BLTs and SLTs may be used as selectingtransistors to control the corresponding NAND strings. In one example,the gate dielectric for BLTs and SLTs may be a silicon oxide layer thatdoes not include a silicon nitride trapping layer. Such configuration,although not necessarily required in every case, may avoid possible Vtshift of BLTs and SLTs during the operations of the memory array in someexamples. Alternatively, the BLTs and SLTs may use the combination ofONONO layers as their gate dielectric layers.

In some examples, the gate voltages applied to BLTs and SLTs may be lessthan 10 V, which may cause less gate disturb. In cases where the gatedielectric layer of BLTs and SLTs may be charged or trapped withcharges, additional −Vg erase can be applied to the gates of BLT or SLTto discharge their gate dielectric layers.

Referring again to FIG. 22 a, each BLT may be coupled to a bit-line(BL). In one example, a BL may be a metal line having the same orapproximately the same pitch to that of STI. Also, each SLT is connectedto a source line (SL). The source line is parallel to the WL andconnected to the sense amplifier for read sensing. The source line maybe a metal, such as tungsten, or polysilicon line, or a diffusionN+-doped line.

FIG. 23 a illustrates a cross-sectional view of an exemplary memoryarray, such as an SONONOS-NAND memory array, along the channel-lengthdirection. Typically, Lg and Ls is approximately equal to F, whichgenerally represents the critical dimension of a device (or node). Thecritical dimension may vary with the technologies used for fabrication.For example, F=50 nm stands for using a 50 nm node. FIG. 23 billustrates a cross-sectional view of an exemplary memory array, such asan SONONOS-NAND memory array, along the channel-width direction.Referring to FIG. 23 b, the pitch in the channel-width direction isapproximately equal or slightly larger than that in the channel lengthdirection. Therefore, the size of a memory cell is approximately4F²/cell.

In examples of manufacturing a memory array, such as the arrays notedabove, the processes may involve using only two primary masks orlithography processes, such as one for the polysilicon (word line) andanother for STI (bit lines). In contrast, the manufacturing of NAND-typefloating gate devices may require at least two-poly processing andanother inter-poly ONO processing. Accordingly, the structure andmanufacturing processes of the proposed devices may be simpler thanthose of NAND-type floating gate memories.

Referring to FIG. 23 a, in one example, the spaces (Ls) between wordlines (WLs) may be formed with shallow junctions, such as shallowjunctions of N+-doped regions, which may serve as source or drainregions of the memory devices. As illustrated in FIG. 23A, additionalimplantation and/or diffusion process, such as a tilt-angle pocketimplantation, may be carried out to provide one or more “pocket” regionsor pocket extensions of junctions that neighbor one or more of theshallow junction regions. In some examples, such configuration mayprovide better device characteristics.

In examples where STI is used of isolating separate memory devices, thetrench depth of STI regions may be larger than the depletion width inp-well, especially when the junction bias used is raised higher. Forexample, the junction bias may be as high as about 7V for programinhibited bit line(s) (unselected bit line(s) during programming). Inone example, the depth of STI regions may be in the range of about 200to 400 nm.

After a memory array is manufactured, a reset operation may be performedto tighten the Vt distribution first before other operations of thememory array. FIG. 24 a illustrates an example of such operation. In oneexample, before other operations start, one may first apply VG=about −7V and VP-well=+8 V to reset the array (The voltage drop of VG andVP-Well can be partitioned into the gate voltage into each WL andp-well). During RESET, the BL's can be floating, or raised to the samevoltage as the P-Well. As illustrated in FIG. 24 b, the reset operationmay provide excellent self-convergent properties. In one example, evenSONONOS devices are initially charged to various Vts, the resetoperation can “tighten” them to a Reset/Erase state. In one example, thereset time is about 100 msec. In that example, the memory array may usen-channel SONONOS devices with ONONO=15/20/18/70/90 angstroms having anN+-polysilicon gate with Lg/W=0.22/0.16 μm.

Generally, traditionally floating-gate devices are not capable ofproviding self-converging erase. In contrast, SONONOS devices may beoperated with converging Reset/Erase methods. In some examples, thisoperation may become essential because the initial Vt distribution isoften in a wide range due to certain process issues, such as processnon-uniformity or plasma charging effects. The exemplary self-converging“Reset” may help to tighten, or narrow the range of, the initial Vtdistribution of memory devices.

In one example of programming operations, the selected WL may be appliedwith a high voltage, such as a voltage of about +16 V to +20 V, toinduce channel +FN injection. Other PASS gates (other unselected WL's)may be turned on to induce the inversion layer in a NAND string. +FNprogramming may be a low-power method in some examples. In one example,parallel programming methods such as page programming with 4K Bytescells in parallel can burst the programming throughput to more than 10MB/sec, while the total current consumption can be controlled within 1mA. In some examples, to avoid program disturb in other BLs, a highvoltage, such as a voltage of about 7 V may be applied to other BLs sothat the inversion layer potential is raised higher to suppress thevoltage drop in the unselected BLs (such as cell B in FIG. 25).

In examples of read operations, the selected WL may be raised to avoltage that is between an erased state level (EV) and a programmedstate level (PV). Other WLs may serve as the “PASS gates” so that theirgate voltages may be raised a voltage higher than PV. In some examples,erase operations may be similar to the reset operation noted above,which may allow self-convergence to the same or similar reset Vt.

FIG. 25 illustrates an example of operating a memory array. Programmingmay include channel +FN injection of electrons into an SONONOS nitridetrapping layer. Some examples may include applying Vg=about +18 V to theselected WLN-1, and applying VG=about +10 V to other WLs, as well as theBLT. The SLT can be turned off to avoid channel hot electron injectionin cell B. In this example, because all the transistors in the NANDstring are turned-on, the inversion layer passes through the strings.Furthermore, because BL1 is grounded, the inversion layer in BL1 haszero potential. On the other hand, other BLs are raised to a highpotential, such as a voltage of about +7 V, so that the inversion layerof other BLs are higher.

In particular, for cell A, which is the cell selected for programming,the voltage drop is about +18 V, which causes +FN injection. And the Vtmay be raised to PV. For cell B, the voltage drop is +11 V, causing muchless +FN injection, as FN injection is sensitive to Vg. For cell C, only+10 V is applied, causing no or negligible +FN injection. In someexamples, a programming operation is not limited to the techniqueillustrated. In other words, other adequate program inhibit techniquesmay be applied.

FIGS. 24 a, 26, and 27 further illustrate some examples of arrayoperations and illustrate the endurance and retention properties of someexamples. As illustrated, the device degradation after a number ofoperation cycles may remain very small. FIG. 24A illustrates anexemplary erase operation, which may be similar to a reset operation. Inone example, the erase is performed by sector or block. As noted above,the memory devices may have good self-converging erase property. In someexamples, the erase saturation Vt may be dependent on Vg. For example, ahigher Vg may cause a higher saturated Vt. As illustrated in FIG. 26,the convergent time may be around 10 to 100 msec.

FIG. 27 illustrates an exemplary reading operation. In one example,reading may be performed by applying a gate voltage that is between anerased state Vt (EV) and a programmed state Vt (PV). For example, thegate voltage may be about 5 V. On the other hand, other WLs and BLT andSLT are applied with a higher gate voltage, such as a voltage of about+9 V, to turn-on all the other memory cells. In one example, if Vt ofcell A is higher than 5 V, the read current may be very small (<0.1 uA).If Vt of cell A is lower than 5 V, the read current may be higher (>0.1uA). As a result, the memory state, i.e. the stored information, can beidentified.

In some examples, the pass gate voltage for other WLs should be higherthan the high-Vt state or the programmed state Vt, but not too high totrigger gate disturb. In one example, the PASS voltage is in the rangeof about 7 to 10 V. The applied voltage at the BL may be about 1 V.Although a larger read voltage may induce more current, the read disturbmay become more apparent in some examples. In some examples, the sensingamplifier can be either placed on a source line (source sensing) or on abit line (drain sensing).

Some examples of NAND strings may have 8, 16, or 32 memory devices perstring. A larger NAND string may save more overhead and increase arrayefficiency. However, in some examples, the read current may be smallerand disturb may become more apparent. Therefore, adequate numbers ofNAND string should be chosen based on various design, manufacture, andoperation factors.

FIG. 28 illustrates the cycle endurance of certain exemplary devices.Referring to FIG. 28, P/E cycles with +FN program and −FN erase may becarried out, and the results suggest good endurance characteristics. Inthis example, the erase condition is Vg=about −16 V for 10 msec. In someexamples, only single shot of erase is needed and verification of statusis not necessary. The memory Vt window is good without degradation.

FIGS. 29 a and 29 b illustrate the IV characteristics of exemplarymemory devices using different scales. In particular, FIG. 29 aillustrates a small swing degradation of the device, and FIG. 29 billustrates a small gm degradation of the device. FIG. 30 illustratesthe retention characteristics of an exemplary SONONOS device. Referringto FIG. 30, a good retention is provided by having less than 100 mVcharge loss for device operated after 10K cycles and after leaving for200 hours at room temperature. FIG. 30 also illustrates an acceptablecharge loss at high temperatures.

In some examples, a split-gate design, such as a split-gate SONONOS-NANDdesign, may be used to achieve a more aggressive down-scaling of amemory array. FIG. 31 illustrates an example of using such design.Referring to FIG. 31, the spaces (Ls) between each word line, or betweentwo neighboring memory devices sharing the same bit line, may bereduced. In one example, Ls may be shrunk to about or less than 30 nm.As illustrated, the memory devices using a split-gate design along thesame bit line may share only one source region and one drain region. Inother words, a split-gate SONONOS-NAND array may use no diffusionregions or junctions, such as N+-doped regions, for some of the memorydevices. In one example, the design may also reduce or eliminate theneed for shallow junctions and neighboring “pockets”, which in someexamples may involve a more complicated manufacturing process.Furthermore, in some examples, the design is less affected byshort-channel effects, because the channel length has been increased,such as increased to Lg=2F−Ls in one example.

FIG. 32 illustrates an exemplary manufacturing process of a memory arrayusing a split-gate design. The schematic diagram is merely anillustrative example, and the memory array may be designed andmanufactured in various different ways. Referring to FIG. 32, aftermultiple layers of materials for providing the memory devices areformed, those layers may be patterned using a silicon oxide structure asa hard mask formed over those layers. For example, the silicon oxideregions may be defined by lithography and etching processes. In oneexample, the pattern used for defining the initial silicon oxide regionsmay have a width of about F and the space between the silicon oxideregions of about F, resulting a pitch of about 2F. After the initialsilicon oxide regions are patterned, silicon oxide spacers may then beformed surrounding the patterned regions to enlarge each silicon oxideregion and narrow their spacing.

Referring again to FIG. 32, after the silicon oxide regions are formed,they are used as a hard mask to define or pattern their underlyinglayers to provide one or more memory devices, such as multiple NANDstrings. In addition, insulating materials, such as silicon oxide, maybe used to fill in the spaces, such as Ls spaces shown in FIG. 32,between the neighboring memory devices.

In one example, the space Ls between neighboring memory devices alongthe same bit line may be in the range of about 15 nm to about 30 nm. Asnoted above, the effective channel length may be enlarged to 2F−Ls inthis example. In one example, if F is about 30 nm and Ls is about 15 nm,Leff is about 45 nm. For the operation of those exemplary memorydevices, the gate voltage may be reduced to below 15 V. In addition, theinter-polysilicon voltage drop between word lines may be designed to beno larger than 7V to avoid breakdown of the spacers in the Ls spaces. Inone example, this may be achieved by having an electric field of lessthan 5 MV/cm between neighboring word lines.

The Leff with diffusion junctions for conventional NAND floating-gatedevices is about half of the their gate length. In contrast, if F isabout 50 nm and Leff is about 30 nm, Leff is about 80 nm for theproposed design (the split-gate NAND) in one example. The longer Leffcan provide better device characteristics by reducing or eliminating theimpact of short-channel effects.

As illustrated above, a split-gate NAND design may further shrink thespace (Ls) between neighboring memory cells of the same bit line. Incontrast, traditional NAND-type floating-gate devices may not provide asmall spacing, because inter-floating-gate coupling effect may lose thememory window The inter-floating gate coupling is the interferencebetween adjacent memory cells when the coupling capacitance betweenadjacent floating gate is high (the space between the floating gates issmall so that the coupling capacitance between the adjacent floatinggates becomes very high such that read disturb happens). As noted above,the design may eliminate the need to fabricate certain diffusionjunctions, and the inversion layer can be directly connected if all theword lines are turned on. Therefore, the design may simplify themanufacturing process of memory devices.

A multi-layer SONOS device is described using and ultra-thin ONOtunneling dielectric. With an n+ polysilicon gate, a self-convergentpositive erase threshold voltage of for example about +3 V is achievedsuitable for a NOR architecture, in which channel hot electronprogramming can be applied for storing two-bits per cell, read using thestandard reverse read method, and erased with hole tunneling erase applyelectric field assisted FN tunneling with a gate voltage of for example−15 volts. With a p+ polysilicon (or other high work function material)gate, a depletion mode device can be obtained having an erase thresholdvoltage less than zero, with a very large memory window with a programthreshold voltage over about 6 volts can be achieved, suitable for NANDarchitecture using electric field assisted FN electron tunneling forprogram and electric field assisted FN hole tunneling for eraseoperations, with a gate voltage during erasing of for example −18 Volts.

FIG. 33 is a graph of change in threshold voltage of a MOSFET having anultra-thin multi layer gate dielectric (O1/N1/O2=15/20/18 Angstroms)versus an number of shots of program disturb bias pulses or erasedisturb bias pulses showing negligible charge trapping in the ONO gatedielectric regardless of the injection mode CHE, +FN, −FN in anexemplary device with a tunneling dielectric.

FIG. 34 is a graph of change in gate voltage versus time under constantcurrent stress in an ultra-thin ONO dielectric capacitor, demonstratingsmall charge trapping under negative gate current stress and indicatingexcellent stress tolerance. The small trapping efficiency may be due tothat the capture mean free path is much longer than the nitridethickness of about 20 Angstroms. This suggests that the N1 layer lessthan 20 Angstroms is desirable. In addition, no interfacial trapsbetween O1/N1 and N1/O2 are introduced during processing in preferredembodiments.

FIG. 35 is a graph of the self-convergent threshold voltage V_(T) as afunction of erase gate voltage V_(G) during the erase process of adevice having an ultra-thin multi layer tunnel dielectric(O1/N1/O2=15/20/18 Angstroms) and having an N+-poly gate. A largermagnitude gate voltage V_(G) results in a higher saturation value ofV_(T) because gate injection is stronger. A high self-convergent eraseis desirable in NOR architectures, because it avoids over-eraseproblems.

FIG. 36 is a graph of threshold voltage versus baking time, for theexemplary device having an N+-poly gate at various P/E cycle counts, forboth erase state and programmed state cells, showing excellent electronretention for the multi-layer BE-SONOS device.

For NAND applications, a depletion mode device (V_(T)<0) for the erasedstate is desired. By using a P+-poly gate, the gate injection is reducedand the device can be erased into depletion mode as shown in FIG. 37.FIG. 37 is a graph of flat band voltage versus time for a multi-layercell (ONONO=15/20/18/70/90 Angstroms) showing that the erase timedecreases with a higher magnitude negative gate voltage. FIG. 37 alsoillustrates that at larger V_(G) (e.g. about −20 volts), gate injectionbecomes significant, resulting in erase saturation at around −1 Volts.

FIG. 38 is a graph of flat band voltage versus time for +FN programmingcharacteristics at V_(G) equal to +19, +20 and +21 volts, for anexemplary device having a P+-poly gate and an ONONO=15/20/18/70/90Angstroms. As illustrated in FIG. 38, a large memory window (as much asabout 7 V in this graph) can be obtained within 10 msec and a 3 V memorywindow can be achieved in less than 200 μsec.

FIG. 39 is a graph of flat band voltage versus program/erase cyclenumber, for a program pulse of +20 volts for 500 micro seconds percycle, and erase pulse of −20 volts for 10 msec per cycle or −18 voltsfor 100 msec per cycle, illustrating the P/E cycle endurance of anexemplary device having a P+-poly gate, showing excellent cyclingendurance. In FIG. 39 a one-shot program and a one-shot erase was usedduring each P/E cycle.

FIG. 40 is a graph of flat band voltage versus stress time, illustratinga V_(G) accelerated retention test with −V_(G) applied for programmedstate and +V_(G) for erased state of the exemplary device having aP+-poly gate. As illustrated in FIG. 40, the small charge loss and smallcharge gain indicate that the direct tunneling is suppressed at mediumelectric field (<4 MV/cm).

FIG. 41 is a graph of flat band voltage versus time, illustrating chargeretention in the charge trapping nitride N2 at room temperature and athigh temperature of an exemplary device having a P+-poly gate inaccordance with an embodiment. As can be seen in FIG. 41, the chargeloss and charge gain are negligible at room temperature. Additionally,more than a 6 V memory window can be preserved even after 500 hours of150 baking. The large memory window greater than 6 volts, and theexcellent retention are very good results for SONOS type devices.

FIG. 42 is a graph of flat band voltage versus time for an erase processin a ONONO device with N2 and O3 layers at 70 and 90 Angstromsrespectively, and the O1, N1 and O2 layers at 15/20/18, 15/20/25 and18/20/18, illustrating that the erase speed of BE-SONOS devices improvessignificantly with the O1 layer thicknesses less than 20 Angstroms,specifically in this example at 18 Angstroms or 15 Angstroms. In fact,with O1 at 15 Angstroms, the erase speed improves substantially, makingerase speeds less than 100 milliseconds and less than 10 millisecondsachievable. For a 15 Angstrom O1 layer, more than 3 volts reduction inflat band voltage (which correlates closely with change in thresholdvoltage) is accomplished in less than 10 milliseconds. As can be seen inFIG. 42, the erase speed is very sensitive to changes in O1. As can beseen in FIG. 42, a decrease in O1 thickness from 18 Angstroms to 15Angstroms results in a dramatic decrease in erase time. Changes in thethickness of O2 generally have a much smaller effect on erase speed.This is because the ONO tunneling is dominated by the O1 layer, whilethe O2 layer is either almost screened (as shown in FIG. 5 c) or iscompletely screened (as shown in FIG. 5 d) during an erase biasoperation.

FIG. 43 is a graph of flat band voltage versus time for an erase bias of−18 volts, in a device having BE-SONOS structure withONONO=15/20/18/70/90 Angstroms. FIG. 43 is a comparison of the erasecharacteristic of the exemplary device having a P+-poly gate and theexemplary device having a Platinum (Pt) gate. Pt has a higher workfunction than P+ polysilicon, that is sufficient to lead to anon-saturated erase as is shown in FIG. 43. The high work function gatematerial can be patterned, for example, by a lift-off process.

As illustrated, some examples noted above, including the structuraldesign, array design, and operation of memory devices, may providedesirable array dimension, good reliability, good performance, or thecombination of any of them. Some examples noted may be applicable fordown-scaling the dimensions of non-volatile flash memories, such as NANDflash memories and flash memory for data applications. Some examples mayprovide SONONOS devices with uniform and high speed channelhole-tunneling erase. Some examples also may provide good endurance ofmemory devices and reduce certain no hard-to-erase or over-erase issues.Also, good device characteristics, such as small degradations after P/Ecycles and good charge retention, may be provided. Device uniformitywithin a memory array may be provided without having erratic bits orcells. Furthermore, some examples may provide good short-channel devicecharacteristics via a split-gate NAND design, which may offer a bettersense margin during the operations of the memory devices.

FIG. 44 is a schematic top view showing a portion of an exemplary memoryarray implemented using thin film transistor structures on an insulatingsubstrate. Referring to FIG. 44, a portion of a memory array 400 isformed over an insulating substrate 401. The portion of the memory array400 includes a plurality of parallel semiconductor body regions 410formed within an insulating layer in the substrate 401 and a pluralityof parallel word lines 420 c between select lines 420 a and 420 b. Theselect lines 420 a and 420 b and the word lines 420 c may be over andsubstantially perpendicular to the semiconductor body regions 410. Thenumber of the word lines 420 c is not limited to number shown in FIG.44. The number of the word lines 420 c may be 8, 16, 32, 64, 128 orother number that is desired to be configured for a memory array.

The substrate 401 may be a silicon substrate, a III-V compoundsubstrate, a silicon/germanium (SiGe) substrate, an epi-substrate, asilicon-on-insulator (SOI) substrate, a display substrate such as aliquid crystal display (LCD), a plasma display, an electro luminescence(EL) lamp display, or a light emitting diode (LED) substrate, forexample. For silicon on insulator SOI embodiments, the substrate 401includes at least one insulating dielectric layer such as dielectriclayer 405 (shown in FIG. 46A) formed over a bulk material 401, such as asemiconductor chip.

Referring again to the embodiment shown in FIG. 44, each of thesemiconductor body regions 410 includes at least one junction regionsuch as the junction regions 412 adjacent to the select lines 420 a and420 b on opposite ends of a continuous, junction-free channel region.The semiconductor body regions 410 include at least one region such asregions 414 between two neighboring word lines. The select line 420 amay be referred to as a block select line and the select line 420 b maybe referred to as a source select line. The junction regions 412 arecoupled to global bit lines or source lines, by contact vias orotherwise (not shown). The select lines 420 a and 420 b are configuredto couple a selected block or band of memory cells to the bit lines andsource lines, when voltages are applied to the select lines 420 a and420 b.

In the illustrated embodiment, the portion of the memory array 400includes a plurality of parallel isolation trench structures 430adjacent to the semiconductor body regions 410, and between twoneighboring semiconductor body regions 410.

Referring again to FIG. 44, the rectangle 402 indicates a cell size of amemory cell, which is basically about twice the width of a wordline 420c, times the sum of the width of a trench 430 and the width of a well410.

FIG. 45 is a schematic cross-sectional view of a portion of an exemplaryarray taken along the section line 2-2 through a word line 420 c of FIG.44, showing a perspective crossing the columns of cells in the array. InFIG. 45, the trench structures 430 are formed between two adjacentsemiconductor body regions 410. A tunneling barrier 310, a chargestorage layer 320, a dielectric insulating layer 330 and a conductivelayer 335 overly and may be substantially conformal over the structureof the semiconductor body regions 410 and the trench structures 430.Detailed descriptions of the tunneling barrier 310, the charge storagelayer 320, the dielectric layer 330 and the conductive layer 335 aredescribed below in conjunction with FIG. 46A.

FIGS. 46A and 46B are schematic cross-sectional views showing stages inan exemplary method for forming an exemplary semiconductor structuretaken along section line 3-3 of FIG. 44.

FIG. 46A is a cross-section taken along the line 3-3 of FIG. 44, showinga single column of cells in an junction-free NAND configuration. Asshown in FIG. 46A, the dielectric layer 305 overlies the substrate 401.The body region 410 is formed over the dielectric layer 305. Thedielectric layer 305 may be, for example, an oxide layer, a nitridelayer, an oxynitride layer, other dielectric layer or variouscombinations thereof. In some embodiments, the dielectric layer 305 maybe referred to as a buried oxide layer as known fromsilicon-on-insulator (SOI) structures. The body region 410 may be asilicon layer, a polysilicon layer, an amorphous silicon layer, asilicon-germanium layer, an epitaxial layer, other semiconductormaterial layer or various combinations thereof. In some embodiments forforming a p-type semiconductor body region, the semiconductor bodyregion 410 may have dopants such as boron, gallium, aluminum and/orother group III element. In some embodiments, the semiconductor bodyregion 410 and the dielectric layer 305 may be formed by asilicon-on-insulator (SOI) process. In other embodiments, the dielectriclayer 305 may be formed by a chemical vapor deposition (CVD) process, anultra high vacuum chemical vapor deposition (UHVCVD) process, an atomiclayer chemical vapor deposition (ALCVD) process, a metal organicchemical vapor deposition (MOCVD) process or other CVD process. Thesemiconductor body region 410 may be formed by, for example, anepitaxial process, a CVD process, a crystallization process, or variouscombinations thereof. In one embodiment, the TFT device has a 60 nmthick poly silicon channel over the buried oxide. The poly silicon is anamorphous silicon (a-Si) layer deposited by low-pressure chemical vapordeposition (LPCVD) process, followed by a low-temperature thermalannealing (600) for crystallization. A multi-layer O1/N1/O2 tunnelingdielectric is used to achieve easy hole tunneling during erase, whileeliminating the direct tunneling leakage during retention. Next, SiNtrapping layer (N2) and top blocking oxide (O3) are grown. A heavilydoped P+-poly gate is adopted to suppress the gate injection during −FNerase. The device has a tri-gate structure as shown in FIG. 45, havingin effect, 3 channel surfaces, one on each side and one on the top ofthe body 410.

The top oxide process has the largest thermal budget. Two top oxide (O3)formation processes are representative, including an LPCVD oxide (HTO)with rapid thermal annealing, and an in-situ steam generation (ISSG)oxidation to convert a portion of the trapping nitride (N2) into oxide.Lower thermal budget processes are preferred to reduce dopant diffusionfrom the select gate junctions. However, the ISSG process may result inbetter endurance characteristics, as we reported in Lai, et al. “AMulti-Layer Stackable Thin-Film Transistor (TFT) NAND-Type FlashMemory”, International Electron Devices Meeting, IEDM, December, 2006,which article is incorporated by reference as if fully set forth herein.Planarization is then carried out by, for example, HDP oxide depositionand Chemical Mechanical Polishing. After forming the bottom-layer TFTdevices, the processes are repeated to form a second and subsequentlayers of TFT devices. Contact etching for the multiple layers can beperformed independently to avoid over etching.

Referring again to FIG. 46A, the semiconductor body region 410 includesa continuous, junction-free channel region 414 between the select lines420 a, 420 b, underlying the word lines 420 c and between the word lines420 c. The semiconductor body region 410 includes at least onecontinuous, junction-free channel region such as regions 415 under theselect lines 420 a, 420 b and the word lines 420 c.

Each of the select lines 420 a, 420 b includes a gate insulator 331 anda conductive layer 336. The gate insulator 331 may be an oxide layer, anitride layer, an oxynitride layer, high-k dielectric layer, otherdielectric material layer or various combinations thereof. Theconductive layer 336 may be, for example, a polysilicon layer, anamorphous silicon layer, a metal-containing layer, a tungsten silicidelayer, a copper layer, an aluminum layer or other conductive materiallayer. The conductive layer 335 may be formed by, for example, a CVDprocess, a physical vapor deposition (PVD) process, an electroplatingprocess and/or an electroless plating process.

Each of the word lines 420 c may include the tunneling barrier 310, thecharge storage layer 320, the dielectric layer 330 and the conductivelayer 335. In some embodiments, the tunneling barrier 310, the chargestorage layer 320, the dielectric layer 330 and the conductive layer 335may be sequentially formed over the semiconductor body region 410.

The tunneling barrier 310 may allow charges, i.e., holes or electrons,tunneling from the semiconductor body regions 410 to the charge storagelayer 320 during an erase operation and/or a reset operation. Thetunneling barrier 310 may be an oxide layer, a nitride layer, anoxynitride layer, other dielectric layer, or various combinationsthereof. In some embodiments, the tunneling barrier 310 may include afirst oxide layer (not labeled), a nitride layer (not labeled) and asecond oxide layer (not labeled) which are referred to as an ONOstructure. In some embodiments, the first oxide layer may be anultra-thin oxide which may have a thickness of about 2 nm or less. Inanother embodiment, the first oxide layer may have a thickness of about1.5 nm or less. In other embodiments, the first oxide layer may have athickness between about 0.5 nm and about 2 nm. The ultra-thin oxidelayer may be formed, for example, by an in-situ steam generation (ISSG)process. The process for forming the nitride layer may use, for example,DCS and NH3 as precursors with a processing temperature at about 680. Insome embodiments, the nitride layer may have a thickness of about 3 nmor less. In other embodiments, the nitride layer may have a thicknessbetween about 1 nm and about 2 nm. The second oxide layer may be formedby, for example, a LPCVD process. In some embodiments, the second oxidelayer may have a thickness of about 3.5 nm or less. In anotherembodiment, the second oxide layer may have a thickness of about 2.5 nmor less. In other embodiments, the second oxide layer may have athickness between about 2.0 nm and about 3.5 nm.

The charge storage layer 320 may store charges such as electrons orholes therein. The charge storage layer 320 may be, for example, anitride layer, an oxynitride layer, a polysilicon layer or othermaterial layer that may desirably store charges. In some embodiments forforming a nitride charge storage layer, the process may use, forexample, dichlorosilane DCS and NH₃ as precursors with a processingtemperature at about 680. In other embodiments for forming an oxynitridecharge storage layer, the process may use, for example, DCS, NH₃ and N₂Oas precursors. In some embodiments, the charge storage layer 320 mayhave a thickness of about 5 nm or more, for example, about 7 nm.

The dielectric layer 330 may isolate the conductive layer 335 from thecharge storage layer 330. The dielectric layer 330 may be, for example,an oxide layer, a nitride layer, an oxynitride layer, an aluminum oxidelayer, other dielectric materials or various combinations thereof. Insome embodiments, the process for forming the dielectric layer 330 mayconvert a portion of the charge storage layer 320 such as a nitridelayer so as to form the dielectric layer 330. The process may be a wetconversion process using O2 and H₂O gas in furnace at a temperaturebetween about 950 and about 1,000 For example, a nitride layer having athickness of about 13 nm may be converted into the dielectric layer 330having a thickness of about 9 nm and the remaining nitride layer, i.e.,the charge storage layer 320, having a thickness of about 7 nm. The wetconversion process is applied for a small initial part of the layer,followed by deposition of the balance of the layer by lower thermalbudget processes for deposition of silicon dioxide, such as a hightemperature oxide HTO process or an in situ steam generation ISSGprocess. In still other embodiments, the dielectric layer 330 is formedover the charge storage layer 320 without a wet conversion process.Various thicknesses of the tunneling barrier 310, the charge storagelayer 320 and the dielectric layer 330 may be used to form a desiredstructure.

The conductive layer 335 may be, for example, a polysilicon layer, anamorphous silicon layer, a metal-containing layer, a tungsten silicidelayer, a copper layer, an aluminum layer or other conductive materiallayer, and combinations of layers of materials. The conductive layer 335may be formed by, for example, a CVD process, a physical vapordeposition (PVD) process, an electroplating process and/or anelectroless plating process. In some embodiments, the conductive layers335 and 336 can be formed by the same process. In some embodiments, thestructure including the tunneling barrier 310, the charge storage layer320 and the dielectric layer 330 may be referred to as a bandgapengineered SONOS (BE-SONOS) structure.

Referring again to FIG. 46A, dielectric materials 339 are formed betweenthe select lines 420 a, 420 b and the word lines 420 c, and between theword lines 420 c. The dielectric materials 339 may include, for example,oxide, nitride, oxynitride and/or other dielectric material. Thedielectric materials 339 may be formed by, for example, a CVD process.At least one dielectric spacer such as dielectric spacers 337 are formedon sidewalls of the select lines 420 a and 420 b. The dielectric spacers337 may include, for example, oxide, nitride, oxynitride and/or otherdielectric material. In some embodiments, the dielectric spacers 337 andthe dielectric material 339 are made of the same material and formed bythe same process.

Referring to FIG. 46B, an implantation process 340 implants dopants intothe semiconductor body region 410 by using the dielectric spacers 337and/or dielectric materials 339 as an implantation mask so as to form atleast one doped region such as the regions 412 to form junctions withinthe semiconductor body region 410. The regions 412 may be referred to assource/drain (S/D) regions of the select lines 420 a and 420 b. In someembodiments, the implantation process 340 may be a tilt implantationprocess, such that the regions 412 may be desirably formed within thesemiconductor body region 410. In other embodiments, the implantationprocess 340 may have an implantation direction substantiallyperpendicular to the surface of the substrate 401 over which transistorsare formed. In some embodiments for forming n-channel transistors, theimplantation process 340 may use n-type dopants such as phosphorus,arsenic and/or other group V element.

Referring again to FIG. 46B, the implantation process 340 does notimplant dopants such as n-type dopants into the semiconductor bodyregion 410 such as a p-type semiconductor body region, because thedielectric spacers 337 and the dielectric materials 339 block theimplantation process 340. Therefore, the implantation process 340 doesnot form source/drain regions at the regions 214 between the selectlines 420 a, 420 b and word lines 420 c. It is also noted that noimplantation process is carried out to form common source/drain regionsat the regions 414 of the semiconductor body region 410 in FIG. 46A.Accordingly, the regions 414 of the semiconductor body region 410 arejunction-free. The dopant concentration of the regions 414 is thussubstantially equal to that of the regions 415, providing ajunction-free, continuous channel region under the select lines 420 a,420 b and word lines 420 c.

FIG. 46C is a schematic cross-sectional view showing an exemplaryprocess for implanting dopants within semiconductor body regions. InFIG. 46C, a patterned mask layer 350 is formed over the select lines 420a, 420 b and word lines 420 c. The patterned mask layer 350 covers atleast portions of the select lines 420 a, 420 b and the word lines 420c. The patterned mask layer 350 protects the regions 414 of thesemiconductor body region 410 from being implanted with dopants of theimplantation process 355. The patterned mask layer 350 may be, forexample, a patterned photoresist layer, a patterned dielectric layer, apatterned material layer that is adapted to be an etch mask and variouscombinations thereof. After the implantation process 355, the patternedmask layer 350 may be removed. The implantation process 355 may be atilt implantation process or an implantation process with a directionsubstantially perpendicular to the substrate 401.

FIG. 47 is a schematic cross-sectional view showing a portion of anexemplary stacked array structure. In FIG. 47, another array structurelayer 357 may be formed over the structure of FIG. 46B. The arraystructure layer 357 may include, for example, a dielectric layer 360formed over the select lines 420 a, 420 b and word lines 420 c. Thedielectric layer 360 may be an oxide layer, a nitride layer, anoxynitride layer, a low-k dielectric layer, an ultra-low-k dielectriclayer, other dielectric material layer, or various combinations thereof.The dielectric layer 360 may be formed by, for example, a CVD process, aspin-on-glass process and/or other process that is adequate to form adielectric layer.

Referring again to FIG. 47, the array structure 357 may further includeat least one semiconductor body region such as a semiconductor bodyregion 365 including regions 367, 368, 369, select lines 370 a, 370 b,word lines 370 c, gate insulators 371, tunneling barriers 372, chargestorage layers 374, dielectric layers 376, conductive layers 380, 381,dielectric spacers 382 and dielectric materials 384, which are similarto the semiconductor body region 410 including the regions 412, 414,415, the select lines 420 a, 420 b, the word lines 420 c, the gateinsulators 331, the tunneling barriers 310, the charge storage layers320, the dielectric layers 330, the conductive layers 335, 336, thedielectric spacers 337 and the dielectric materials 339, respectively,described above in conjunction with FIG. 46B. It is noted that the arraystructure layer 357 is formed over the structure of FIG. 46B. The dopedregions 412 (shown in FIG. 44) are subjected to the same thermal cyclefor forming, for example, the dielectric layer 360, the semiconductorbody region 365, the select lines 370 a, 370 b, the word lines 370 c,the tunneling barrier 372, the charge storage layer 374, the dielectriclayer 376, the conductive layer 380, the dielectric spacers 382 and/orthe dielectric materials 384 described above in conjunction with FIG.47. The doped regions 412 may extend toward the select lines 420 a, 420b so as to form the doped regions 412 a. The extending regions 412 a mayhave a dimension “a” larger than dimension “b” of the regions 367.

It is noted that the exemplary structure of FIG. 47 does not have commonsource/drain regions formed between the select lines 420 a, 420 b andword lines 420 c and between the word lines 420 c. Even after more thanone thermal cycle, the regions 412 a may not extended and be adjacent toother junctions or doped source/drain regions. Accordingly, the issuesof the short channel effect and the leakage current within the memoryarray may be desirably prevented.

FIG. 47 merely shows an exemplary embodiment including 2 stacked arraystructures. The number of the array structures, e.g., the arraystructure 357, is not limited to two. Two or more array structures maybe formed over the structure of FIG. 47 in order to achieve a desiredmemory capacity.

FIG. 48 is a schematic cross-sectional view showing an exemplary processfor generating an inversion layer in a semiconductor body region.Referring to FIG. 48, a voltage “V” may be coupled to the selected wordline 420 c. In some embodiments, a space “S” between two neighboringword lines, such as 420 c and 420 d, or 420 c and 420 e, may be about 75nm or less. In exemplary embodiments, the space S is 30 nm or less. Dueto the small space, the voltage “V” applied to the word line 420 c maybe coupled to and generate an inversion layer in the regions 411 withinthe semiconductor body region 410 between two adjacent word lines, suchas 420 c and 420 d, or 420 c and 420 e, and in the region 411 a beneaththe word line 420 c. The inversion layers 411, 411 a may serve as S/Dterminals of array transistors. In some embodiments using NAND-typestructures, voltages are applied to each of the word lines 420 c-420 eand may invert and/or generate the inversion layers between two adjacentword lines 420 c-420 e and select lines 420 a, 420 b. Therefore, thearray transistors may desirably function without heavily doped S/Djunctions within semiconductor body region 410.

Reset

In some embodiments, a reset operation may be performed to tighten theVt distribution first before other operations of the memory array. Forexample, voltages are applied to and turn on the select lines 420 a and420 b. Prior to other operations, a voltage of about −7 V may be appliedto the word lines 420 c-420 e and a voltage of about +8 V may be appliedto the semiconductor body region 410 as shown in FIG. 48. The voltagesapplied to select lines 420 a and 420 b are higher than the voltageapplied to word lines 420 c-420 e. The voltage drop of the word lines420 c-420 e and the semiconductor body region 410 may be desirablyportioned into the gate voltage into each word line and semiconductorbody region. In some embodiments, the memory array may be charged withvarious voltages. The reset operation may desirably reset the cells ofthe memory array. In some embodiments, the reset time is about 100millisecond. In some embodiments for resetting the memory array, thememory array may include n-channel BE-SONOS devices withONONO.apprxeq.15/20/18/70/90 angstroms and have an N+ polysilicon gatewith Lg/W 0.22/0.16 μm.

Programming

In some embodiments for programming cells of the memory array, a highvoltage, e.g. between about +16 V and about +20 V, may be applied toword line 420 c to induced channel +FN injection. In some embodiments,the high voltage is about +18 V. A voltage, such as about +10 V, may beapplied to other pass gates, i.e., unselected word lines 420 d and 420 eto induce inversion layers in the NAND string. Semiconductor body region410 is substantially grounded. Charges, such as electrons, can beinjected into the charge storage layer of word line 420 c. In someembodiments, the +FN programming may be a low-power programming. In someembodiments, parallel programming methods, such as page programmingmethod with 4 K Bytes cells may desirably increase the programmingthroughput to more than 10 MB/sec. The total current consumption can beabout 1 mA or less. In some embodiments, a voltage, such as about 7 V,may be applied to other bit lines to avoid program disturb. The voltageapplied to the bit lines may raise the potential of the inversion layerto suppress the voltage drop in the unselected bit lines.

Erasing

In some embodiments, the erasing operation may be similar to the resetoperation. A voltage of about −7 V may be applied to the word lines 420c and a voltage of about +8 V may be applied to the semiconductor bodyregion 410 as shown in FIG. 48. The voltage drop of the word lines 420 cand the semiconductor body region 410 may be desirably portioned intothe gate voltage into each word line and semiconductor body region.

Read

In some embodiments for reading the memory array, the selected word linemay be raised to a voltage, such as about +5 V, that is between anerased state level (EV) and a programmed state level (PV) of a memorycell. Other unselected word lines may serve as the “PASS gates” so thattheir gate voltages may be raised a voltage higher than PV. In someembodiments, the voltage applied to the pass gates is about +9 V. Insome embodiments, a voltage of about +1 V is applied to thesemiconductor body region 410.

The structures and methods for forming the structures described above inconjunction with FIGS. 44, 45, 46A-46C and 47 may be applied in anyNAND-type flash memories with various cell structures such as flashmemory with polysilicon floating gate.

Exemplary Embodiments

Following are descriptions of exemplary junction-free BE-SONOS devices.In some embodiments, the device has a poly pitch of about 0.15 um. Afterpatterning the hard mask of poly, an oxide liner can be formed tofill-in the poly space, e.g., about 70 nm or more, followed by the polyetching to define the final poly space. It is found that the device maybe free from abnormal poly short or line-end breaking. The narrow space(S) between the sidewalls of the oxide liner can be accuratelycontrolled by the liner oxide thickness.

The conventional junction implantation can be performed after the polyetching. In embodiments of junction-free devices, shallow junction canbe saved and spacers can be kept. Oxide spacer can be fill-in the narrowspace between word lines. A tilt-angle implantation can be carried outto form the junction outside and adjacent to the array. Due to the thickpoly gate blocking the implantation, the array center is not subjectedto the tilt-angle implantation and is junction-free. It is noted thatthis process is desirably compatible with conventional NAND process. Noadditional mask is required.

Following are the descriptions of electrical characteristics ofjunction-free devices. The devices are configured with a 16-WL NANDarray. The ONONO structure, e.g., O1/N1/O2/N2/O3, has dimensions ofabout are 13/20/25/60/60 Å, respectively. Å

FIG. 49A illustrates the effect of various p-well doping. A lighter welldoping provides larger electron density, enabling more current flow.FIG. 49B illustrates the effect of space (S). When S is increased, theelectron density is slightly decreased at the space, leading to lowercurrent.

FIG. 50 is a figure showing the measured initial IV curve of exemplaryn-channel devices. Junction-free devices can have similar subthresholdbehavior as conventional junction devices. It is found that the draincurrent is slightly lower of the junction-free device than that of theconventional junction device. It is also found that larger space (S)shows slightly lower current. FIG. 51 shows that a heavier well dopantconcentration can increase the Vt of the junction-free device, which isconsistent with the simulation shown in FIG. 49A.

FIGS. 52A-52B are drawings showing +FN ISPP programming and −FN erasing,respectively. Junction-free devices can have similar electricalcharacteristics as conventional junction devices. The reason may beattributed to +/−FN injection being governed by the intrinsic ONONOproperty, and irrelevant with junctions.

FIG. 53 is a drawing showing electrical characteristics of an exemplaryP-channel BE-SONOS NAND having a stack structure similar to theN-channel BE-SONOS NAND described above in conjunction with FIG. 50. InFIG. 53, it is found that the junction-free device has larger Vtdifference and smaller current than those of the conventional junctiondevice. The reason can be that the conventional junction device is notdesirably optimized and has large Vt roll-off effect.

For the p-channel NAND, the program/erase voltage polarity should beopposite to that of the n-channel NAND. FIGS. 54A-54B are drawingsshowing −FN ISPP programming and +FN erase of an exemplary p-channelBE-SONOS NAND. From FIGS. 54A-54B, it is found that −FN ISPP programmingand +FN erase of an exemplary p-channel BE-SONOS NAND can be performed.

FIG. 55 is a drawing showing endurance of exemplary n-channel devices.In FIG. 55, the junction-free device does not have substantialreliability degradation, whereas the conventional junction device does.

FIG. 56 is a drawing showing IV curve of exemplary TFT BE-SONOS devices.To understand the impact of thermal budget, a post thermal anneal with850 for 20 minutes to simulate the thermal budget during 3D integration.In FIG. 56, the TFT device shows similar electrical characteristics asthose after thermal anneal. This result is desired for 3D integrationprocesses, since junction-free are much less sensitive to thermalbudget.

FIG. 57 is a drawing showing simulations of exemplary junction-freedevices having various technology nodes (F=half pitch of poly), andhaving same spaces (S=20 nm). In FIG. 57, it is found that thejunction-free devices Vt-roll-off can be desirably controlled. This maybe attributed to the larger effective channel length in thejunction-free devices than in conventional junction devices. In FIG. 57,the effect of programmed states are also simulated. For devices havingsmall F, the programmed Vt shift is reduced.

The reason can be that the device channel length is short such thatfringing field degrades the gate control capability. It should be notedthat the junction-free devices are desired for charge-trapping devices.For embodiments of floating-gate devices, the small space (S) can inducemore FG-FG interference.

The foregoing disclosure of the preferred embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. It will be appreciated bythose skilled in the art that changes could be made to the embodimentsdescribed above without departing from the broad inventive conceptthereof. It is understood, therefore, that this invention is not limitedto the particular embodiments disclosed, but it is intended to covermodifications within the spirit and scope of the present invention asdefined by the appended claims.

What is claimed is:
 1. A method for forming a semiconductor structure,comprising: forming a plurality of first parallel semiconductor bodyregions with a first dopant type over a substrate; forming a pluralityof first parallel word lines between a first select line and a secondselect line, the first word lines, the first select line and the secondselect line being over and intersecting the first semiconductor bodyregions in an array of cross points; forming a first tunneling barrier,a first charge storage layer and a first dielectric layer between thefirst semiconductor body regions and the first word lines; forming firstdielectric spacers on a sidewall of the first select line and a sidewallof the second select line; forming first source/drain (S/D) junctionswith a second dopant type adjacent to the first select line and thesecond select line; forming a second dielectric layer over the firstword lines; forming a plurality of second parallel semiconductor bodyregions with the first dopant type over the second dielectric layer;forming a plurality of second parallel word lines between a third selectline and a fourth select line, the second parallel word lines, the thirdselect line and the fourth select line being over and substantiallyperpendicular to the second semiconductor body regions; forming a secondtunneling barrier, a second charge storage layer and a third dielectriclayer between the second semiconductor body regions and the second wordlines; forming second dielectric spacers on a sidewall of the thirdselect line and a sidewall of the fourth select line; and forming secondsource/drain (S/D) regions with the second dopant type adjacent to thethird select line and the fourth select line; wherein the regions in thefirst semiconductor body regions between two neighboring first wordlines and the regions in the second semiconductor body regions betweentwo neighboring second word lines are junction-free.
 2. The method ofclaim 1, wherein forming first dielectric spacers includes forming firstdielectric materials between two neighboring first word lines.
 3. Themethod of claim 1, wherein forming first source/drain (S/D) regionsincludes using the first dielectric spacers as an implantation mask. 4.The method of claim 1, further comprising forming a plurality of trenchstructures adjacent to and parallel with the first semiconductor bodyregions.
 5. The method of claim 1, wherein forming the S/D junctionscomprises: forming a patterned mask layer overlying at least portions ofthe first and second select lines and the first word lines; andimplanting dopants of the first dopant type into the first semiconductorbody regions by using the patterned mask layer as an implantation mask.6. The method of claim 1, wherein forming the first tunneling barriercomprises forming a tunnel dielectric structure having multilayer orcomposite composition with a hole tunneling barrier height at aninterface with the semiconductor body, and a hole tunneling barrierheight spaced away from the interface that is less than the holetunneling barrier height at an interface.
 7. The method of claim 1,further comprising forming an oxide layer between the substrate and thefirst semiconductor body regions.
 8. The method of claim 1, whereinforming second dielectric spacers includes forming a second dielectricmaterial between two neighboring second word lines.
 9. The method ofclaim 1, wherein forming second source/drain (S/D) regions includesusing the second dielectric spacers as an implantation mask.
 10. Themethod of claim 1, wherein forming the first S/D regions comprisesimplanting dopants with the second dopant type into the firstsemiconductor body regions by using the first dielectric materials as animplantation stop layer so as to prevent implanting the dopants withinthe first semiconductor body regions between two neighboring first wordlines.
 11. The method of claim 1, wherein the method does not include animplantation process for forming common source/drain regions within thefirst semiconductor body regions between the first word lines.
 12. Amethod for operating a semiconductor structure, the semiconductorstructure comprising a plurality of parallel semiconductor body regionsover a substrate; a plurality of parallel word lines between a firstselect line and a second select line, the word lines including aselected word line and a plurality of unselected word lines, the wordlines, the first select line and the second select line being over andsubstantially perpendicular to the semiconductor body regions; and atunneling barrier, a charge storage layer and a dielectric layer betweenthe word lines and the semiconductor body regions, wherein thesemiconductor body regions include at least one first region adjacent tothe first select line and the second select line and second regionsbetween two neighboring word lines, wherein the first region has adopant concentration higher than that of the second regions and whereinat least one of the second regions is junction-free, the methodcomprising: applying a first voltage to the first select line and thesecond select line; applying a second voltage to the word lines, thefirst voltage being higher than the second voltage; and applying a thirdvoltage to the semiconductor body regions to reset the semiconductorstructure, the third voltage being higher than the second voltage. 13.The method of claim 12, further comprising: applying a fourth voltage tothe selected word line; applying a fifth voltage to at least one of theunselected word lines to induce at least one inversion layer between theword lines, the fourth voltage being higher than the fifth voltage toinject charges within the charge storage layer; and grounding one of thesemiconductor body regions, which is coupled to the second regionsadjacent to the selected word line.
 14. The method of claim 13, furthercomprising: applying a sixth voltage to the selected word line, thesixth voltage being lower than the fifth voltage; applying a seventhvoltage to the unselected word lines, the seventh voltage higher thanthe sixth voltage; and applying an eighth voltage to the groundedsemiconductor body region to read a state stored in the charge storagelayer, the eighth voltage being lower than the sixth voltage.
 15. Themethod of claim 12, further comprising: applying a sixth voltage to thefirst select line and the second select line; applying a seventh voltageto the word lines, the sixth voltage being higher than the seventhvoltage; and floating the semiconductor body regions to erase thecharges stored in the charge storage layer.
 16. A method for forming asemiconductor structure, comprising: forming a plurality of firstsemiconductor body regions; forming a plurality of first word lines,each of the plurality of first word lines overlying a channel region ineach of the first semiconductor body regions; forming a first tunnelingbarrier, a first charge storage layer, and a first dielectric layerbetween each of the first word lines and a corresponding channel regionin each of the first semiconductor body regions, wherein the firstsemiconductor body regions between one of the first word lines andanother one of the first word lines are junction-free; forming aninter-dielectric layer over the first word lines; forming a plurality ofsecond semiconductor body regions overlying the inter-dielectric layer;forming a plurality of second word lines over the second semiconductorbody regions; and forming a second tunneling barrier a second chargestorage layer and a second dielectric layer between the second wordlines and the second semiconductor body region.